Do you know what type of 3-5-2 formation you want to use? Depending on that you could need different types of players.
I'll say some very general info on positions. CB needs to be good at defending, preferably bigger and stronger players. Things I look for most in CBs is strength, marking and tackling.
LB/RBs (also known as fullbacks) are not used in the 3-5-2 formation but you usually want them to be fast and be good at tackling and slide tackling.
CDMs are players that need to have high stamina as they often run alot. Strength is also a good thing for these players so that you can win the ball back in midfield. I think strength, stamina and tackling are the highest priority here.
CAMs are midfielders that need to be able to attack and focuse much less on def. Best skills would be passing, vision, and even scoring.
CMs need to be somewhere between CDMs and CAMs. They need to be able to help on both ends. Again stamina is huge for these guys. I like CMs fit the saying "the whole is greater than the sum of the parts." I like finding ones that aren't great or terrible at anything.
LM/RM these players play out wide and are usually good at crossing and having speed is important. They need to be able to track back and defend at times(especially in 3-5-2) to cover attacking fullbacks or the lack of having fullbacks in the formation. Key stats would be crossing, speed, and maybe decent defensive stats for the 3-5-2 formation.
LW/RWs (wingers) are much like LM/RMs in that speed is a key factor. The big differnce is that wingers are much more attack minded and often have really bad defensive stats. Crossing is again a big one but finishing is more important for wingers. I for example love to have wingers cut in to the middle and shoot. For me the key stats are speed, crossing, finishing and stamina.
STs need good finishing but there are many types of STs. Some have speed and need to have the dribbling to get into space and score. Some are bigger guys and use headers to score from crosses. The type of ST you need depends completely on play style more than probaly any other position. If you play with a 2 ST formation, I suggest having one of each.
CF is kinda halfway between ST and CAM they pass better than a ST and shoot better than a CAM.
GKs are pretty obvious :P
Hope this helps and really hope it makes any sense
If you want more detailed info on positional needs then ask about specific positions and I'll try to answer. I'm not the most knowledgeable person when it comes to soccer but I've come a long way and always want to help anyone else in their journey