No, miles aren't deducted until you actually ticket, up to 3 days later. When I was searching for flights I put 4 different sets of UA tix on hold, with confirmation numbers. Only one was charged to my account, 2 days later when I ticketed it.
The only option is to put them on hold for 3 days, then re-put them on hold at 12:01a 3 days later, 7 times in a row. Or just hope they're there 20 days from now.
(hold is until midnight on the 3rd day, not 72 hrs)