You really can’t say if they can justify it or not. Only they can. If I can save 15 minutes per day, that would more than pay for $12/month. Time is your most valuable asset.
If you are a 3-person SaaS startup and your team is spending 15+ minutes a day just scheduling meetings among themselves, and need additional software just to manage that process, I'd say that's a problem in itself.
If that's internal meetings, absolutely, that's an issue. But I think a company that size that's trying to iterate on their offering should be booking a lot of meetings to talk with customers and potential customers, and that could easily take 15 mins a day in my experience.
Someone said this in a comment above, but in a 3-person SaaS, everyone works sales. It's likely they're scheduling a huge number of meetings with external folks.
I have 0 meetings with anyone that works with me. We’ve essentially eliminated all internal meetings. At least scheduled recurring ones.
I do, however, have roughly 10 meetings per week. These are with potential clients, existing clients (for feedback), external consultants (legal), investors, etc.
We are a startup, and as such, we all wear 10 different hats. Which means doing sales, admin, business, etc.