We are trying to wire 3 baseboard heaters (all 240V: 750W, 1000W and 1500W, in parallel) attached to a douple pole 20amp breaker. We tried 12 gauge 2 wire first and they were working but not much heat. So someone told us maybe we'd only wired them using 120V(? ). So we now have re-wired them using 3 wire, all different ways and no luck. We have since learned from reading here that 3 wire isn't need as baseboard heaters require no neutral so we ignored and capped the neutral at both ends, still no luck!
Winter is coming and we are in a mad panic to figure this out as the new heaters are our only source of heat.
Could someone please explain in laymans terms exactly how to properly hook them up to each other and to the double pole breaker as well as how we can test or know that we are running 240volts and not 120volts.

Many thanks and appreciation in advance, we have become very frustrated!