Right. If they don't need to be 3 individual stereo mixes, that lets you get away with 2 transmitters and 3 receivers—for 1 stereo mix on transmitter #1, and 2 mono mixes on transmitter #2.
With a Carvin EM900, that would set you back $399 for two transmitter/receiver packages, and another $233 for an additional receiver. (Buds come with the systems, but not with the extra receiver. Carvin sells extra buds at ~ $100, but I'd be inclined to buy elsewhere.) If money isn't a big constraint, you could get three stereo systems for not much more scratch than a single PSM900. (Stereo separation really helps me hear singing pitch while keeping the volume blend for backing vocals.)
Whether that makes sense for you depends on whether the PSM900 buys you any useful performance advantage over the EM900 for your gig situations. I've found the Carvin's meet my needs perfectly. (Good clarity, plenty of system gain, and no drop outs. Caveats: I'm using non-Carvin ear pieces, just b/c I already had a few sets; and I've only had the system for a year, so I can't speak even anecdotally about long-term reliability.)
That said, I'm not crowd surfing hundreds of feet from my transmitter, nor subjecting the receiver to Gwar-like stage conditions. YMMV.