Remember first of all that margin accounts in future markets are different from margin accounts in equity markets in the sense that, in future markets you don’t use the margin account as leverage (e.g. put down x% and borrow the rest). Instead, the margin accounts are rather used as a buffer, given that futures are settled on a daily basis (aka mark to market) in case the position incurs too many losses. That is, at the end of the day, you either receive the gains you made or you have to pay for the losses you made.
Remember also that:
Rising (falling) prices, of course, benefit (hurt) the long and hurt (benefit) the short. So since we are short here, any increase in the price is a loss to the trader.
Let’s look at your example:
Initial margin is 5 and maintenance margin is 3, this means the investor puts down $5 per contract, thus $50 at the beginning of the trade.
First day, the trader loses $3 per contract (so he loses $30 total), ($100-$103=-$3), Margin account is at $50-$30=$20.
Second Day: The trader gains $7 per contract (thus 70 total), $103-$96=$7. Margin account is at $20+$70=$90
Third day: Trader loses $2 per contract (20 total), ($98-$96=$2). Margin account is at $90-$20=$70.
( Note: The losses on the first day would trigger a Margin Call, and the margin account would need to be increased up to the initial margin again, but we ignore the margin calls here as requested in the question )
By the way, the whole thing is described in a nice and short paragraph in _ Reading 58, 4.1.2. Futures and in Example 9 Reading 46. _
Hope this helps.