These are your three choices
- Put target creature on top of its owner's library
- Jeskai Charm deals 4 damage to target opponent
- Creatures you control get +1/+1 and gain lifelink until end of turn
The first mode is essentially Griptide, a 4-mana card (3U) printed in Theros & Dark Ascension. Forcing a creature away and forcing an opponent to draw is nice. Likewise, you can use it to save one of your own creatures, as long as you don't mind having it as the next card you draw. For 3 mana, that's a really good deal.
The second mode is much like the first and most popular mode on the popular burn spell, Boros Charm: deal 4 damage to a player. You can also redirect that damage to a planeswalker - unless the defending player has a Leyline of Sanctity on the board - because to direct the damage to a planeswalker, you have to be able to target the opponent first. This mode is a bit redundant in Modern, as why would you want to pay one more mana for it?
The third mode gives creatures you control +1/+1 and lifelink until end of turn. The quick boost in power is good, but the lifelink is even better. It's hard to say how often this mode would be used in Modern, but it's a great option to have late in games. In a deck that runs Monastery Mentor, being able to give the Mentor and his tokens an additional +1/+1 and lifelink in addition to their Prowess triggers seems good enough to play this card.
All three of these modes are solid enough, and Jeskai (Red/White/Blue) is a popular archetype in Modern. Here’s one Jeskai Midrange deck that saw some success on Magic Online, and this Jeskai Geist of Saint Traft deck from a Modern PPTQ. A few decks have played one copy in either the main board or the side board. Being in three colors isn’t really a problem in Modern.
Still, Boros Charm is strictly better for the 4 damage, due to being one mana less. The Griptide effect isn’t bad, and is useful for Tempo, but it would easily be the rarest used mode in that format. The last ability is probably the one that would be the best use for this card in Jeskai Modern decks, since they typically run Monastery Mentor or other creatures, such as Restoration Angel, that would happily take the boost and lifelink.
All that being said, is Jeskai Charm good enough for Modern? It certainly fits into what those colors are trying to do in Modern, and as a one-of is a nice utility card. However, it probably won’t ever be a staple in the format.