As has been stated previously, if you're riding in the big chainring and the big cog on you're cassette, you're putting alot of stress on the components. This will cause the chain, chainring and cassette to wear out prematurely. You can get almost the same gear inch by using your middle chainring and a different cog in the back. As for the rear derailleur, yes there is a long cage deraillleur and it is made for a triple setup. It is what I use on all of my bikes. Sorry, everyone. but out here in Seattle with all of ours hills a triple, just makes climbing them easier.
I agree with Grampy, a worn chain and rear cassette may have allow for the combination of big and big. And with you new chain it isn't. If, you go on ebay, you can get a new triple rear derailleur, fairly inexpensively.