RQ3 is a very different animal from any previous (RQ1/2) or post (Mongoose 1/2) edition of the game. It was the most complex by far, with the most subsystems in the mechanics, including a really whacked sorcery system (even the designers did not use it!)
I sold all my RQ2/RQ3 material for BIG bucks 5-6 years ago, including my RQ3 deluxe boxed set, perfect bound complete book, and many of the AH Glorantha products. The RQ3 rulebooks are very difficult to find for a reasonable price these days, unless you get lucky on ebay. Be aware there is a USA Avalon Hill RQ3 "Standard" rules box (blue), which 5-6 years ago was going for super cheap and was common as it was a condensed version of the most basic RQ3 rules (IOW, no-one wanted it), but the prices have gone up on these too. You may also try the Amazon aftermarket sellers- I believe I saw (nearly) complete Deluxe boxed set contents there several months ago.
I know of no PDFs for sale for RQ3.
The MRQ2 system, IMO is far better than the RQ3, or MRQ1 system, and for the time being readily available (Mongoose has lost the license, but will be publishing the exact same rules system under another name). The Current BRP rules may suffice but keep in mind they are just the "generic" BRP system, kinda like GURPS or HERO.