Setting up a swarm - amp question

I currently run two REL subs using the high level input, one from each channel of my integrated amp.  I want to add a third sub, again using high level input but summed across both channels (the same way REL has one setup a single subwoofer.  Will this cause any issues to the amp.  How many subs using high level connections can an amp tolerate?
I have no direct experience with REL subs, but would expect them to have negligible effect on the load that the amplifier sees when connected as you describe.

So I think you can hook up as many as you can afford and/or shoehorn into your room before your wife files for divorce. 

Right. All powered subs, the way the speaker level input works, the first part of the circuit is a resistor, typically about 10 k Ohm, that drops the speaker level voltage down to line level. So the additional load on the amp is negligible.

I had to learn about all this earlier this year when I added my own Swarm type distributed bass array. My integrated had no line out. So I modified it by converting the speaker out to line level with this simple circuit. 

Duke of cours is also absolutely right about "as many as you can afford and/or shoehorn into your room". I now have five. Two sealed and two ported 10", plus one Talon Roc. The more, the better!