Yes, there will be some signal loss to each leg--you are sending the current to 3 legs instead of 2.
The RF output of a 622... is somewhat amplified IIRC so it should be enough for up to 100' RG-6.
I'd recommend a small amplifier to help if there is a problem.
I combined 2 722s and antenna (3-way) to feed an R.S. 10dB 4-way amp sent to the 2 722s, a remote 622, the TV, and other old equipment. The Antenna and Dish feeds are ATSC and the others are NTSC but none may be on the same channels. Keep at least one channel blank between the signals--can be hard with cable--I chose OTA 60 62 64 66. (BTW, a 10dB amp and a 4-way reversed combiner give say <1/4 or ~-14dB of the signal from this amp.) This looks like a loop but isn't.
What is the new signal will it interfere with the cable signal? You are sending a small part of the Dish and the other signal out on the cable line. Generally, cable puts a fair amount of isolation on each line but you could cause problems for your neighbors.
-Ken