bobjobb,
You only need to derate the branch circuit or overstate the load by 25%, not both, as you indicated in your last post. 3 x 1500 watts equals 4500. 4500 watts @ 125% equals 5625 watts. At 240 volts, that would be a little less than 24 amps. Therefore, 2 circuits are required.
Approaching it from the reverse. How many feet of baseboard can be connected to a single 2 pole, 20 amp breaker? (250 watts per foot). 20 amps at 80% equals 16 amps. 16 amps times 240 volts equals 3840 watts. Divide that by 250 watts and you get 15 feet of baseboard per 20 amp circuit breaker. (a useful number to remember).
Could your job get by on 3 five foot baseboard heaters rather than 3 six footers?