ERJ-135,
One stripe is an instrument rated private pilot. Who may or may not have a multi-engine rating (there is no additional stripe for multi)
Two stripes is a commercial pilot. Who may or may not have a commercial multi-engine rating (there is no additional stripe for multi)
Three stripes is a CFI. Who may or may not have CFII or MEI. (there are no additional stripes for CFII or MEI)
What you saw was most likely a "route" flight, in which two multi-engine rated private pilots fly an "airline style route" to build multi-time. The commercial pilot (2 striper) you saw, may have entered the program with his ratings up to commercial-multi, but wanted to do the route program, so they teamed him up with a private pilot.
Any more questions, just ask...
The Turk.
P.S. where is your FBO? just curious