Three weeks is when you wean.
However, your babies should already be eating solid foods. Do they eat pellets, hay, and vegetables/greens? Was the mother eating before she passed away? Are the babies with the father? If so, they will learn from him how to eat and drink from the water bottle.
I would weigh them daily (see
Weigh Weekly from
Guinea Lynx :: A Medical and Care Guide for Your Guinea Pig ) for awhile to make sure they are gaining weight and not losing. If they aren't gaining, then handfeeding may be needed. There is information on feeding pups (baby guinea pigs are called pups) here--
Feeding Pups .
You'll need to determine their sexes so you can separate boys from girls at three weeks (and no longer!). Here are some tips for finding out their sexes--
Sexing Pups .
Pet store guinea pigs are often missexed. They often have mites as well. You can find information on mites at
Guinea Lynx :: A Medical and Care Guide for Your Guinea Pig as well.
I'm sorry you lost the mom, and good luck with the babies.