Imho it's like this

Steel: Very longlasting if it has been build 'old school' (read lugged) and comfortable. There are also steel frames which are build like alu frames are build (without lugs), they also have the comfort but lack the 'long lasting' factor. They only negative side to steel (compared to alu and carbon) is weight.

Alu: If not combined with carbon is stiff, not so comfortable and may suffer from microcracks early on in life. However the combination with carbon (front and rear) makes the bike more comfortable since the carbon acts as a cushion for the vibrations, the frame will last longer, downside it that you have glue edges where the carbon is fitted to the alu, this is a potential weakness. Another advantage is weight.

Carbon: As comfortable as steel since it absorbs the vibrations, strong, really light but may not be so long lasting as steel. It is weak in direction the carbonvibers is not woven in, so a crash with the bike could have severe consequences. I have seen a couple of girls who have carbon frames which actually broke in 2 during a crash in a group bike race. When you have such a crash there is allways someone falling or even riding over you frame, both times the frames broke. And carbon is also the most expensive alternative of the three.

My advice would be stick to steel (if you don't care about the weight) or alu with an alu rear-end but with a carbon fork (if you do want weight to count). And invest some extra money in good high-end components.