When you play a 343, you gotta be careful of the position of your players in every play phase cause, if you don't, you won't be able to exit the build-up phase effectively. My tip is to start watching some matches you're not performing in Full match and maybe get some principles by youtube channels like Football made simple to understand what you're doing.
I have a bit of experience with 343 tho and maybe i can help you. IF won't probably stay in the half-space early enough. They will be isolated a lot playing in a wider position. Try to set them to stay narrow but this won't work as i saw by previous testing. Try using one of those roles with stay narrower: trequartista, wide-target man, AP and not working perfectly but could be fine RMD.
The one more thing you could observe is if you wanna play a lot of crosses or not. For example, right side: you're playing with two players trying to wide the game and making crosses whenever they can. Is this what you want? This won't lead to possession tho.
One more thing you gotta be careful about the players with take more risks on. They will try risky first time passes quite often, especially if you raise up mentality.
Please, do not share only the screen of your tactic but try to send some in-match screenshots too and explain what you're searching for. If we tell you, change this, this and this, even if we are right, you won't learn anything