I'm running a 50-42-30t on my triple cranks, the shifting between these is not smooth, the chain takes a moment to find its place. The 50 and 30t ring are Specialized and the 42 is a Sugino. Can I just rotate the middle ring on the crank to get these in sync or is the Sugino ring not compatible with the Specialized rings ?

I would replace the middle ring with a 39 that has ramps and pins to work in a triple.

They're pretty compatible.

What do you mean "the chain takes a moment to find its place"? What derailleur and shifter do you have, and how well are they adjusted?

FWIW: the previous iteration on my Gold Rush recumbent had Deore LX trigger shifters, a FD-R443 derailleur, and an Ultegra crank with Sugino (24), Shimano (42), and SR (53) chainrings. Despite all this mechanical bastardization, it worked perfectly.

The ramps and pins are shifting aides for Index shifters, But I've gotten along with out them for decades

with friction shifting and using the terrain, to allow the load on the chain to be lessened when I need a big Upshift.

like 3rd ring up to the Middle ring at the crest of a Hill..