A translation company has assigned me a project without asking me to sign a SLA in the beginning, after finishing the project, I have been asked to sign a SLA...
Yes, this often happens. It's not the correct procedure, but sometimes the project manager (PM) will forget to send you the agreement, especially if he's in a hurry to get the translation done. He then remembers about it and assumes/hopes that you will be okay with it.
I suppose it's possible that some PMs might do this deliberately, i.e. send the agreement only after the translation was delivered,to force the translator into signing it if he wants to get paid, but strictly speaking they have to pay (for that one job) even if you don't sign the agreement.
The SLA doesn't mention anything neither about translating part time freelancers, nor even termination of the SLA, it only set the rules for delivering projects and penalties.
The agency doesn't really care if you're a full-time or a part-time translator. It makes no difference to them. The agreement applies to all jobs that you do for the client, but you are not forced to accept any jobs from them. You can sign the agreement, and then (if want to) refuse any or all future assignments from them. If you want to cancel the agreement, you can either (a) write to them to inform them that you're cancelling the agreement or (b) simply stop accepting work from them. If you don't do any work, then the agreement doesn't apply to any of your work because there is no work that it can apply to.
If you do cancel the agreement, then the agreement still applies to all previous assignments that you have completed for them up to that point, but it does not apply to any new assignments from them (although they are unlikely to send you any further assignments if you cancel the agreement).
Usually, the agency contract is different from an employment contract. An employment contract applies all the time, and it forces you to do a certain amount of work. The agency contract applies only to the specific assignments that you have accepted individually.
What penalties does the contract mention?
I read it many times and it doesn't mention any thing about my rights but mostly theirs.
Yes, that is normal. Agencies' lawyers tend to forget about the rights of translators. But if you are concerned about your own rights, then you can also draw up an agreement that you want the agency to sign. As long as nothing in your agreement clashes with anything in their agreement, it should be okay.
...and I think the 3 years period is to keep the rate unchanged all this time.
No, I don't think so, unless the rate is mentioned in the agreement. You are free to raise or lower your rate at any time. In fact, the agency is also free to offer you more or less than your current rate, even during the next 3 years.
[Edited at 2016-09-21 07:46 GMT]