Tri-interquine - Three programs that output each other in a loop
Python 3, 50 bytes



s='s=%r;print(s%%(s,%i*2%%7))';print(s%(s,1*2%7))



Try it online!

The last expression goes from 1*2%7 to 2*2%7 to 4*2%7 then back to 1*2%7.