What if I sign a work contract and then regret before starting?

I got a job offer from company A to start in ~3 months from now, and they want me to sign their contract within one week. Now, I am also awaiting for an offer from company B, which already expressed they want to hire me but, due to internal bureaucracy, they need to approve my hiring in a meeting before creating the contract, and this will happen a few days after the deadline from company A. I am more interested in B, but the offer from A is not bad. So my options are:

1. Sign with A and tell B I don't care anymore. Let's call this the safe option.
2. Reject A and await for B. Let's call this the risky option, since there is a little (but nonzero) chance that B will not send me the contract, then I may end up with nothing.
3. Sign with A and, should I get the contract from B, then sign with B and cancel A.

My question is about option 3. What are the implications of signing with A and two days after this I want to cancel this contract, before even starting to work (which will happen in some months from now). For sure they will hate me, but other than that, from the legal point of view, what are my obligations? Do I have to pay a fine? Do I go to jail? I am living in Zurich and the two companies are also in Zurich. I'm sure I am not the first one in this situation.

What does it say in the contract you already have about cancellation by either side?