Recording 3 hardware synths live into Sonar - setup?

I've been using Sonar for years, but 99% of it with soft synths, so all my mixing etc has been in-the-box.  Now I need a solution to handle recording in Sonar X2 3 hardware synths playing live.   My current soundcard can only take 2 synths (4 instrument INS for 2 x stereo).   Can anyone suggest a way I can handle this in a reasonably cost-effective way?   I was thinking a digital mixer, with the 3 synths plugged in there (analogue) and then hook the mixer's digital out into my soundcard, which I presume records as 1 stereo audio track into Sonar?   Seems expensive though, unless someone knows some cheap-ish digital mixers to recommend?   Any other options?