Pretty much all Chapters have a mix of suits, and Mk3s are actually still prized for their reinforced frontal armour (and brutal appearance). The Raven Guard are slightly worse equipped than you'd expect for a First Founding, being mostly equiped with Mk6 rather than Mk7 armour. No reason they wouldn't have a higher number of older suits (though Mk3s are for brutal, frontal assault - not their forte).
The Marines Malevolent are using older equipment, both armour and bolters, according to the Salamanders books. The Red Scorpions have a real mix of armour types, as the Forge World Chapter, and there's no reason for an invented Chapter to not be poorly equipped.
More fleet-based Chapters will be better at boarding actions - the best examples I can think of are the Minotaurs and the Astral Claws/Red Corsairs.
Last edited by Eremite; 11-13-10 at 12:42 AM.