Q: Can the MidNite Solar MNPV6-250 Combiner Box mount two 20 amp 300vdc and one 20 amp 600vdc breakers?

A: No, this combiner is only rated for 300 volts.

Q: So could four 20 amp 300 vdc breakers work? The first string of 371 volts could be halved...making it 185.6 on each leg and the other two would be 278.4 each.

A: This may work if you have at least 2 charge controllers, otherwise, your amperage (80A) will exceed the OCPD rating of any of the 300VDC combiners. You may be better off purchasing a high voltage combiner instead, such as one of the MNPV6HV or MNPV8HV combiners.