Counseling preference filling doubt

My rank is 19.5k, and I want Mechanical at the main campus. My question is, how do I fill the preference order?
If I place
1)Mech
2)Chem
3)Civil
And they allot Civil to me in R1, and I let go of the seat and continue to R2, will I lose my 10k for R1 because I forfeited a seat?
Also, will they only check preferences 1 and 2 this time while ignoring the 3rd civil one?
Similar for R3.
I want Mechanical and am fine with getting into Chem or Civil so that I can get Mech in the intra round or later branch change into Mech, so I want to keep my options open for Chem and Civil up until R3, in case I don't get Mechanical by then.
How do I fill my preference order?