"if else if" like condition in data formula in Scribe

Hi Experts, 

I hope we have many scribe experts here in the community.

I am very new to Scribe - i am in a situation where i have to check three field values which can either be 0 or 1 or null. if all source fields are "0" i have to set target to "0", if all are "1" i have to set target to "1". otherwise if any field is null or not matching is criteria it has to be null.

I am able to check something like below :

IF(AND(S1=0, S2=0,S3=0),"0","")

but how do it check other value from these fields? Like and IF , Else IF condition? 

your help is much appreciated.