oh congratulations sarah! The average litter size is described as 6-8 but of course this is the average, so it may be that this time, Mum just gave birth to a smaller litter. As Souffle says, her fertility may have been affected by the time of year.
I wouldn't worry about it, it is much better I think to have a smaller litter that all make it, rather than a larger litter where quite a few of the pups dont. We previously had a litter of 13 but by 3 weeks only 3 survived, which was quite distressing, but nature's way.
I was talking to quite a large scale breeder recently who reported that she had had lots of hamsters either not produce, produce smaller litters, or had lost a lot of pups, and she mentioned the season as being a possible reason.
Hope your 3 darlings grow well and fast, and when their eyes are fully open we would love you to introduce them to us!