Hi mate
I reckon the most important thing to make an attacking 4-3-3 work is to have a fluid system
and to pick players with good stamina, work rate and pace attributes.
My advice is to make sure in your 3 in midfield, you have a defensive midfielder completely devoted to roaming in front of your back four snuffing out attacks and playing the simple passes to more creative midfielders.
In him, you want good work rate, positioning, tackling, stamina, strength and concentration.
Depending on how effective your defensive midfielder is, you might want some attacking fullbacks
that will exploit the space on the wings, put crosses in for the strikers and interrchange with the central
midfielders. If you set your fullbacks to an attacking duty, i reccommend you make sure:
a) the fullbacks have good physical attributes/ can track back well
b) your defensive midfielder is good at holding up attacks until cover is assembled
c) your club either has a big squad, or plays few midweek matches- its difficult to play at a high tempo
consistently without players being rested sufficiently
otherwise, your side will be very vulnerable to pacy counter attacks
As for the central midfielders, in this system you need them to be talented allround. They need to be able to get back occassionally to support the defence, pass the ball well, arrive late in the box for a shot in a Lampard-esc style. They must be as mobile as possible.
For the strikers, it's important to have a target man- someone like Andy Carroll. The good thing about having 3 strikers is that the 2 support strikers will confuse the standard flat back-four defence as they won't know who to mark. As long as your target man can hold the ball up well, you want your support strikers to be pacy and have good off the ball movement as well as the obvious finishing and technique attributes.
Hope this helps!
Gabriel