Those specs are nearly identical, so that panel can be safely put in parallel with the new one.
Assuming you mean that your 120 W panels were put in *series*, then that combined 240 W can also be safely put in parallel with the two 300 W panels. If all three panel circuits are in parallel to the same charge controller, each of the three must be individually protect with a fuse or circuit breaker, no more than 20 A.
It sounds like you have more than one charge controller. What models? Which panels are wired to which controller?
Yes, I've installed 2 solar charge controllers - both same specs 5A - 30A 12/24V Model Ecco-LD-24v-30A. I've combined the 120W and 1 x 300W solar panel with 15A circuit breaker and the remainder 300W panels to the other controller also with 15A circuit breakers.
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