Ykhjuy

It takes 1 to 3 weeks but I don't know what to do with my current job that I don't want to lose it, if I failed in the test. It's an on site test and the office is in my city. They told me it's about how you deal with our team and the test tasks that we assign to you.

I need to know how to deal with it professionally.

I've never heard of a "pre-employment" test that involves you being on site, carrying out tasks for 1-3 weeks. The only pre-employment tests I've ever heard of outside of interviews have been tasks that can easily be completed outside of working hours, therefore without your current employer knowing, and shouldn't take more than an evening or two to accomplish (at a maximum.)

Seems like a red flag in my book. The rather odd, and disruptive nature of this is enough that if it were me, I'd just send a polite email declining the offer.

Sounds like you'll be doing actual work for free.

Stick with your current company or find a better offer somewhere else.

Pre-employment tests should include solely short tasks that have been "done a million times" and are considered common knowledge within your profession.

That amount of work should only be agreed to if they pay you for your time as it most likely is not a test.

Let them know that you won't have the time to do such a lengthy test.
Either thank them for their effort and inform them that you'll have to regretfully decline or ask if they could arrange for a shorter test, depending on wether you want the job or not.