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I freakin' love the look of the mkiii armor, the medieval knight look really works for me. Anyway, i just got a kit of the mkiii space marines and im wondering if i can use them in 40k? theyre just a tactical squad that I'd be painting blue and such.

Yes it's just an armour type, for power armour there are no rules.
The terminator armour has different rules which were made available in he angels of death supplement.

Rules wise there is no difference between the armour marks so nobody is going to care what mark you want to use.