I'm not sure why the designers are so dedicated to making it difficult to get an extra arcane slot. An extra ally slot is more powerful, yet Charisma is way cheaper and more efficient than this card. It is true that a lot of characters would really like to have an extra arcane slot, but that is because the arcane slot limit is kind of an annoying inconvenience that would be cool to circumvent, not because it is some vital part of game balance that would be super powerful to overcome. Yes, it would often be worthwhile and convenient to play a third spell, but not at the cost of nearly doubling the cost of a spell (is that third spell you are playing really worthwhile when it costs an extra three resources and a card?). And on top of that this card still has a significant downside that it sits in your hand useless until you actually need to play a third spell. Again, Relic Hunter lets allows you to break the accessory limit as a permanent (!) for no cost other than 3 XP, so why does this card have to be so expensive? I’d think it was still a bit pricey, but more reasonable, if it had a resource cost of 2. A resource cost of 3 just seems mean for a card that does nothing but permit you to then pay for another card which is probably not even one of your best two spells (since those are in your first two slots).
You have 1 additional arcane slot, which can only be used to hold a Spell or Ritual asset.
This card really is the Bandolier of spells. Great if you're running a spell-heavy deck, but this is not necessarily a good thing, because Bandolier itself has some issues in regards to its playability; issues that are not unlike other cards that can hold slots such as Daisy's Tote Bag or Leo's Mitch Brown. These cards do nothing by themselves, but rather enable more space for other cards. The issue is when you draw all those cards.
Draw Sign Magick too early without filling out your spells slots and it's effectively a waste of 3 resources. Draw Sign Magick too late in the scenario and you'll find it hard to justify paying the cost to open up AND fill the additional arcane slot. This timing pressure makes it hard to really value Sign Magick in all situations.
The good thing about Sign Magick as compared to the 'additional slot' cards is that it is fast. This really helps to offset the pressure that comes from playing it. Additionally, the 3 cost of this ritual plays very nicely with Uncage the Soul by choosing to pay the action to play it in exchange for the resource cost. That's some great flexibility in how you pay for these things.
I think this card is an exciting addition for a sealing Mystic, as it gives them the ability to have a Seal of the Seventh Sign, 2 Protective Incantations, and a Chtonian Stone on the board all at once. It can also be used to turn the new Enchanted Blade into basically a two handed weapon if you really need those Arcane slots.