I have a pumpkin that's been putting out male flowers for about 3 weeks with no females yet. I'm worried I might not have any male flowers open when the females do bloom. Can I save some male flowers in the fridge or freeze the flowers/pollen for later use?
Pumpkin plants normally put out female flowers when the plant is large enough to support the pumpkin.
Pollen can be frozen, and it's best done when dried first. How long it lasts though depends on the variety of plant unless you have the resources of the seed vaults available to you. It won't hurt to try saving the pollen. Collect the pollen first thing in the morning as viability drops off during the day.
The pollen doesn't last more then a day in the fridge so that's not worth attempting.
And for information on hand pollination, http://pumpkinnook.com/howto/pollen.htm