??? About Litter Training

My kittens are now 3 weeks and 2 days old. I still see mom licking their bottoms and have yet to see any little 'kitty droppings'. When will they be ready to try out a litterbox? Should I wait until I 'see' some poop?
I have been letting them roam free in the room they are in and at this point have the floor covered with sheets.

Any advice on this? They are in a room with carpet, I cannot keep them in a bathroom and so would like to TRY to get the litter box training done as easy (for the babies) and as quickly, as possible....

So long as Mom's litter box is in the room where everyone is confined, she will teach them when she is ready for them to go in the box. In the meantime, just be patient. It will come.

Are they eating any solid food yet??? The mother will lick the kittens poo away until they start eating solid food. It's makes it taste bad and the mother will no longer clean them. That is when they will start using a litter box. I used the top of a shoebox and filled it about 1/2" with litter.(I lined it of course so the cardboard wouldn't absorb any liquid) Once the kittens were eating things other than mama's milk, they used the litterbox. I would always put them in it to make sure they noticed it. Before they start using it they will either ignore it or play in it. Mine did the latter.LOL