Worth taking fighter for tower shield and crit then?
Well... You don't gain much from not taking a 3rd (or 4th) class in the long run, so a few little extras are nice to have from a level 30 perspective, even though not necessary at all. I wouldn't spend more than a level or two on a 3rd/4th class before epic, anyway.
You don't need to be a fighter in order to take improved critical, if that's what you mean. Fighter's extra feats give you more room to take it, though. It's not a particularly strong feat for a stormlord, but it's nice to have if you have all the basics covered and nothing better to take.
Will i get a debuff on xp with 3 active classes?
Depends on the race, the classes, and how many levels you take on each. Stormlord is a prestige class, and those are ignored as far as XP penalties go. If your other classes are only fighter and cleric, you won't have to worry about XP penalties if you're a human, half-elf, dwarf, genasi or grey orc.