Would the Yamaha RX-V663 able to drive three JTR Tripple 8 without any additional amplifiers?

It depends on how loud you listen and how far you sit from the speakers. They are very sensitive speakers but many people do not realize how much power is actually needed for clean low distortion sound. Again, it depends on the volume and your room setup(seating distance). It also depends on if the yammie was made to handle below 4 ohms. The JTR's are rated at 4 ohms which mean that is an average, they will go above and below.

This is the Yamaha RX-V663 specs but I don't see anything on the Ohms specs.

http://www.yamaha.com/yec/products/productdetail.html?CNTID=567571&CTID=5000300&ATRID=1020&DETYP=ATTRIBUTE

There's a wealth of info available in the V663 thread (http://www.avsforum.com/avs-vb/showthread.php?t=995796&highlight=yamaha+v663).