You can always tie grounds together with cables, or use a metal shunt from each negative terminal to the earth (green terminal) then they'll all be implicitly connected together (but don't do this if you need them to be floating supplies).
I like using 3 single-output supplies. Put them side-by-side and they take up about as much space as a triple-output, and it's easy to just pick one up and replace/transport it, etc. Electronix Express has some inexpensive models:
$85 for 30V/3A isn't bad
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