I asked this recently but my question wasn't very specific so this time I think I have a clearer question. 

What current bookshelf would be better match to the old RC3 and RS3? I think the it's between the RB10, RB51, RB61, and RB81. 

To match the output of the RC-3's dual 6.5" woofer's you would need the RB81. THey are both 97 db efficient and will handle 150 wpc. Both will comfortabley play below 80 Hz. RB61 would work if space was an issue. I would not go smaller and expect a match.

An excellent match would be the RB-5s, which I have in Cherry but aren't going to sell for a bargain basement price. I purchased them to use as Rears for the Bedroom but my wife thought 7.1 in there sounded too intense.

At the same time I bought a pair of RB-75s to use as Rears in my main HT (and already using them as mains in the bedroom) but feel like the RB-5 could have worked out just as well in that capacity. I agree with Colterphoto1 if you aren't interested in buying used, spend the extra money and get a pair of the RB-81s. :-)

The local dealer recommended the RB61 over the RB81 because the size was more of a match and they thought the 81 would overpower the center. I was skeptical but I gave it a try and the RB61 seem awesome. I ended up staying up way, way, way too late watching movies I've seen tons of times already just to enjoy the new balance in the system. The real test was watching a movie with the better half and at the end she said "Did you notice that I didn't ask you to turn it up once?" which means the balance is much better.

As a side note, the RB61 are amazingly small. I couldn't believe there were two in the box.  I have yet to test audio but we rarely use the living room for audio these days.