I have a 2 yo, a 3 yo and an 8 yo. My 8 yo stays up later and the really super cool stuff (microscope etc) comes out after bedtime. My 2 & 3 yo like to be like their sister so I have a special color book for each of them that they ONLY get when my older dd has bookwork and the little ones want to do the same thing. Playdough also only comes out while my oldest is working. My oldest likes to teach things to her little sisters, but your kids are closer in age so modified activities for each of them will probably work best.
For counting and numbers, give each of them counters, sized appropriately or edible. If you're working on forming letters & numbers, give the 16mo a paper with the letter/number on it and some crayons, look over and say 'great job' every so often, when the dextarity comes you can help direct the actual writing, but for now, scribbling will work. Your 3 yo can trace #s drawn in highlighter (WAY easier than drawing dashed lines) and your 5yo can practice forming them on hs own. Paint with pudding, play with a big bucket of rice and some measuring scoops, funnels and bowls. get some plastic kid scissors that don't cut anything and keep the 16 mo on your lap with hers while the older two practice on old magazines.
Lessons at this age should be short and fun. They learn FAR more by 'doing' than they do sitting. Sort toys as you put them away for the night (let's put the trucks away first. Now let's do the little people. Put all of the blue blocks over here....) count things as you put them away (we used 1, 2, 3, 4 cushions from the couch in our fort), Make Glop, oobleck or edible playdough- the 5 yo can mix ingrediants, the 3 yo can pour into the bowl and the 16 mo can hand them to you. For word recognition, label everything you can with a picture and a word. Craft projects can often be modified - if you're making valentines for family or vetrans for example, let the 3&5 yo use glue or glue sticks and give the 16 mo clear contact paper and torn up tissue paper or construction paper. Coloring w/ markers? give the 16 mo the Color wonder ones - also good for finger paint.
do you get Family Fun? There's a section in there with ideas for tots, and they have a separate 'sister' magazine designed specifically for toddlers/preschoolers. The website has most of the same stuff. If you pick something for the littleone, sometimes it's easier to make it harder for the older one rather than simplifing a more comnplex project.
EXPECT messes - and involve the kids in the clean up. And to whoever said they consider it a sucess if the little one doesn't eat the craft supplies - you owe me a new monitor! That's so true, I laughed and stupidly had a drink in my mouth.