We have a Coleman 3400a816 electric furnace. Only one or two of the three elements are working.Do these elements need to be replaced of are there other things that could cause this problem?

You need to make sure that you are getting 240V to the furnace. If not, you have a bad breaker, burned out wire or loose wire to the power side. If you are getting 240, check the continuity of the elements with an ohm meter. Make sure the power is off and connect the ohm meter across the element terminals. I don't know what the factory ohmage is but it should probably be less than 100-200 ohms. If it doesn't read at all, the element is bad. Yes, they periodically burn out and have to be replaced.