The mediant chord: iii
The mediant chord not used that often in the major keys for songwriting and is found more often in the minor keys. This doesn’t mean that we should avoid it completely and I find that the iii chord can add some interesting colour, especially in a bridge. iii rarely moves to V and generally leads to the chords vi, ii and IV.
Some nice progressions using iii are iii-vi-ii-V, iii-ii-V and iii-IV-V. Experiment and see what you can come up with.