You cannot be a member of more than one third order at a time. It is a vocation, like any other and it is considered that you are making a lifelong commitment when the time comes that you make your permanent profession. This is typically after 5+ years of formation which would mean that you had been living and studying that spirituality for the entire time.
Up until then, all of your professions are temporary so you would be free to leave at the end of each block of time which you professed for. For example, in the Dominican Laity, after the initial 6 months of inquiry, if I am accepted by the Chapter and the Order, I will make a 1 year profession which is the Novitiate year. After that year, if i am invited by the
Chapter and the Order, I can make another temporary profession which I believe is for two years. Following that, another two year profession can be offered and accepted. Then, your permanent profession is offered. However, those who request it can do another temporary profession for a year if they request it and the Chapter Moderator and Provincial Moderator approve it. That would give someone well over 6+ years of discernment.
The other option, which is thankfully rare, is for someone who is permanently professed to request to be released from their promises. I am unsure what the process is for that entirely, but I know it goes to the Provincial for some sort of adjudication.
I gather from reading your posts on the subject that you are feeling pulls in more than one direction at the moment. That is fine, there is absolutely nothing wrong with that. Relax, take your time, study, PRAY, and contemplate each direction and something will come to you.
One thing you may consider is seeing if the various orders you are looking into do spiritual retreats. Spending a few days living in that world will likely give you an insight to spirituality from each order’s perspective that you would not otherwise have. It could be a valuable part of your discernment process.
Good luck and God Bless!