Sead Kolasinac is the 23-year-old left-back who arrived at Arsenal at the opening of summer transfer window. His contract with Schalke expired, so we snatched him up on a free transfer. We all know that Arsene Wenger wouldn't miss out on brining a decent player who happens to be free, and the fans were wondering if the Bosnian international would be an important member of the first team at all!
However, Kolasinac's player characteristics and some interviews from the Boss made me believe that not only Kolasinac will be an important first team player, but also that Arsene Wenger will stick to his tactics from the last two months in the previous season - we will have a three-man defense again!
Sead Kolasinac Player Characteristics

Sead Kolasinac changed several German clubs in his youth career, before coming to Schalke as an 18-year-old in 2011. He made a quick breakthrough to the first team in 2012, and became an indispensable member of the squad in 2013.
Kolasinac is well known for his strength and composure - he is 183 cm tall and weighs 85 kg, and is a complete opposite to the light Nacho Monreal, who has only 72 kg. It's no wonder that Sead has the nicknames that he got - 'The Destroyer', 'The Tank' and 'The Bosnian Hulk'.
He brings a strong physical presence to the field, and his work-rate is also high. Therefore, Arsene Wenger believes that he will quickly adapt to Premier League. Kolasinac is also known for his tough but timely tackles, and he is also very dominant in air duels against his direct opponents, wingers, and backs.
Kolasinac lacks dribbling skills, and his technique is way behind Arsenal's standards, but he improved a lot in the previous season, and it seems that Wenger believes he can upgrade his game even further.
The Reason Behind Kolasinac's Arrival

The Bosnian featured for Schalke on three positions in the last season - apart from playing left back, which is his primary position, he ended up playing more games as a left midfielder, 15 in comparison to 13, and he also played as a central defender in two games, to cover for his injured teammates.
During the time he played as a left midfielder, he made an impressive improvement of his attacking skill set - Kolasinac scored two goals and added three assists while playing in that position. He was one of the best Schalke players in that period, and it is clear that Wenger wants to use him as a wing-back and exploit both his physical dominance and defensive awareness as well as his upgraded offensive skill set.
Also, Wenger recently made a hint that he is very pleased with Monreal's performances in central defense, so everything should be clear - Monreal will stay in the club as a central defender, while Kolasinac will cover the vast area down the left flank.
The Emerging of the new Defense System

Arsenal was moderately successful after switching to a three-man defense in the finish of the season. We failed to qualify for the Champions League, but we won a comforting FA Cup and became the most successful club in the competition. Our defense looked better it that period, and it seems that the Boss will stick with a three-man defense system.
If we keep all players, Nacho Monreal, Laurent Koscielny and Shkodran Mustafi should be the standard three at the back. Monreal is, of course, a classic left back, and he will be the nearest player to left wing back Kolasinac in this formation, while Mustafi played as a right back in his career too so that he will be the man to the right. Koscielny will take the central position. Their replacements will be Gabriel, who will rotate with Monreal, Holding, who will rotate with Koscielny, and Chambers, who will rotate with Mustafi if we don't send him to another loan spell.
Down the left flank, Kieran Gibbs should cover Sead Kolasinac, but I might see Gibbs leaving the club. He is consistently failing to break into the first eleven, and he might have had enough of it. Realistically speaking, Gibbs is one of the best English left backs, and numerous teams from the middle of the table would give him a place in the first eleven. If he leaves, Wenger will probably give a chance to some youngster on that position.
Down the right flank, we should watch Hector Bellerin and Alex Oxlade-Chamberlain, although they are wanted by numerous clubs. I believe that Wenger won't sell them unless we get the offer we can't refuse. I don't see Barcelona going on a money splurge for Bellerin, although they need a right back, but we will see what happens until the end of the summer.
The Verdict on the New System

Sead Kolasinac might be an excellent addition to Arsenal's squad; there is no question about that. He is strong, he is young, he is willing to work hard and has a significant upside, but Arsene Wenger can't hope for much better results if he doesn't upgrade his team enough.
We have many alleged transfer targets, including the talents such as Kylian Mbappe, but I'm skeptical about it. The Gunners went through many disappointments in the past few years - after all the fuss, we usually landed some youngster who made less than twenty senior team appearances in his career, or some free agent, such as Kallstrom or Flamini.
For this system to work, we will need high-profile reinforcements, especially in attack, and we will have to finally solve the problems which leave us with ten or more injured players at the same time every single season.
The current team might adapt to the new 3-4-3 system since we already have the players capable of covering more than one position. However, we shouldn't expect that a team which failed to secure the place in Champions League in the previous season will fight for the title in the next.
Arsenal must be the team that is the title contender in every single season, but for that, we will need much more than these cosmetic changes.