Silver sharks are extremely timid and easily frightened (I dont know about columbians but I read they like to be in groups too - as should silver sharks), and in a 29g tank there's nowhere for them to hide - feeding them is going to be a problem, like someone else said - Tetra Prima is your best bet (cichlid pellets are for cichlids - hence the name lol), or sinking pellets of some description. But your common plec will probably get to them before they do - so it really is worth thinking about returning them and getting some fish suitable for your tank - as all three will outgrow yours very quickly anyway.
Silver sharks are shoaling fish too - if they aren't in a shoal they're going to be even more stressed and timid and even less likely to eat. There's a good chance they could starve without company, as they wont have the confidence to go and grab that food before the plec gets to it. You either need to get at least 4 more silver shakrs and seriously upgrade your tank (min 6ft x 2ft x 2ft but preferably bigger), or return them asap. And I'd return the other two also - because as I said earlier, they are quick growing and will get waaay too big for your tank. Min tank size for a common plec is 4ft x 18" x 18".