Should I make partial repayment to my HDB loan now if I intend to sell my flat in the next few years and buy a resale, BTO or EC?

It depends if you are able to generate returns from the money you intend to pay your house to get more than 2.6%. if you can do it, it is a way to abitrage and earn. E.g. if the money can return 4% gains, you net off with 4 - 2.6 = 1.4 % returns more than the amount of money used to pay the loan.

If not, by doing lump sum repayment, you save on interest on these few years of paying the loan.