Liam is a bit harder because - due to his job + Cody he can only be found at certain places during the weekends during the day/afternoon before school closes.
For the Liam/Mick Beach scene you have to advance Mick's scenes quite fast (so that you would have seen him at the beach) with enough time to have another weekend morning to trigger the scene. And still enough time to trigger the jealousy scene between them if you want that too>_>
Jealousy requires that they both have high romance/melting (and spoiler: will force you to choose between them and refreeze/lock the other).
Both of these scenes have a cut off of the day Osiria asks who you like so it's usually easier to trigger one chain of jealousy events at a time than try to raise the levels of the 3 boys simultaneously. (Also because watching one will lock one of the guys and thus make another combination impossible)
Hope this helps!