I would honesly look more at limits and sequencers than elements. Not saying you couldn't have elements out, but in order to make a huge difference you need multiple elements out, it's always more likely you have one part taking out multiple elements than multiple bad elements.
If you look at the above picture there are 6 colored circles in a U shape on the left side of the picture. Those are limits which control idividual elements. Again as those only turn off one at a time I wouldn't think they would be your issue. To the left of those limits (sort of in the middle of the U) is a smaller black circle, that is also a limit. According to the above schematic it appears to shut off control voltage to 3 elements. With the furnace on check across that limit (leads on each side of the limit) if you read 24v your limit is stuck open which will cause 3 elements to not come on.
If that reads no voltage next I would check your sequencers. They are the to black switches just to the right of the U. They turn on 3 elements each, so one going bad can greatly affect the heat being put out by your furnace. There are 6 bigger wires hooking from the elements to the breakers then there are 2 control wires going in the bottom. Check across the 2 control terminals if you read 24v check across the high voltage terminals, if they read 240v the sequencers aren't turning on the elements.