The srd probably won't have monsters such as the mind flayer, beholders, displacer beasts, slaad, etc. that were excluded from the 3.0 srd, and the MM is supposed to be better functional layout than the 3.0 one.
DMG supposedly has different discussions about gaming, some epic level campaign running advice, and spell blast templates for miniatures use, plus campaign specific stuff such as the red wizard prc.
The PH, well it gets used as an at game reference book more than any other.
You should be able to get along fine with just the srd though.