[WillieJohnstonsGreenhouse]

1) Playing with 3 centre backs is a good way of giving your team possibly a spare man at the back as well as a spare man in midfield and 2 strikers upfront. The 3-5-2 for example gives a spare man at the back and a spare man in central midfield which is a great way of playing against a normal 4-4-2. However, playing with 3 centre backs is not useful against teams playing with 1 striker. A 3-5-2 up against a 4-3-3 for example doesn't give a spare man in midfield, leaves 2 centre backs not marking anyone, it can cause havoc against teams who play with flexibility and leaves the 2 wing-backs 1v1 against the opposition wingers.  

2) In modern day football, most teams now play variants of 4-2-3-1 and 4-3-3. The days of most teams playing a basic 4-4-2 like you described are gone. There are still teams who can play 4-4-2 successfully (Albion showed it under Pulis) but generally 4-4-2 is slightly outdated in it's set-up. 4-4-2 has been such a popular formation for the past few decades that every type of 4-4-2 with individual tactics has been worked out by coaches both present and past.

3) Coaches in modern day football have had to learn to be adaptable. Using just 1 formation all the time is never always a good idea because different matches need different formations and tactics. For example, you wouldn't play a 4-4-2 away at Barcelona just like you wouldn't play with 3 centre backs either. For a team like Barcelona, a 4-1-4-1 or 4-3-1-2 formation is usually a good bet to match them up in midfield. On the flip side, if you were playing against the bottom of the league team who are a playing with 3 centre backs and 2 wing-backs (Hull and Wigan in recent years) then playing a 4-4-2 could work because you have 2 strikers against their 3 centre backs. A lone striker can easily be outnumbered, outmuscled and outthought by 3 centre backs.

Sorry about the above answers being a little long but I hope they answer your questions Wille.

Cheers Josh, top answer, knew I could count on you -   now where's the paracetamol
(and less of the old fart drewks  )