In the rules, it states that you must roll AT LEAST three hearts to earn an evolution card. If you are able to roll/use card(s) to garner, for example, 6 stars, can you apply the 'at least' statement to mean per heart, or does AT LEAST indicate that a maximum of 1 evolution can be earned per turn?
No, you may only draw one Evolution per set of rolls.
The rulebook states:
If a monster ends their rolls with at least 3 [hearts], they draw the top Evolution of their deck
Firstly, the term "at least 3" indicates that any number above 2 will match the condition, thus 3 or 6 or anything between will give you the same result.
Secondly since the card effect happens at the time that you end your rolls, only one card can be on top of your Evolution deck at that time.