I've assumed you mean a romantic type of love.
I think it depends on the expectations you and each of the other people have of the relationship you share with them.
Casual dating is like going out with friends - you like/care about each other, but have not said you'll date only this one person. More serious going together usually starts when both people want and expect their bf/gf to be exclusive to him/her. So, to decide if it's cheating to date one person and tell another person you love him, maybe you could think about the following questions.
Is it a casual dating relationship with both, and you have begun to have feelings for one of them and thus you told the second one you love him? Is it a serious, long-term, exclusively dating only one person, yet you've said "I love you" to another?
If dating others vs exclusivity has never been discussed in either relationship, perhaps you could raise the issue with both and work on what their and your expectations are, and then decide if you say "I love you" to one, while still dating the other. I guess the idea I'm trying to get across is that all three of you have some expectations and any or all of you could be hurt if those expectations are not stated up front and then met in the relationship. You might have to make a hard choice: one or the other, or be left with neither...
:hug: