How do i train my 3yr old to stay in his bed and sleep all night?

i have baby twins and they wake up less thanmy toddler but life is torturous with someone always wanting something there is no sleep.he used to be ill alot from nursey and no matter how many times i have got him sleeping through he would get another bug and wake up!.i stopped nursery and he goes to a childminder and is hardlt evr ill now.i have relentlessly taken him back to bed and when ill i would lie by his cot if poss.i cant stand him cryin all nihght at a baby gate,can anyone help?