Ok so set Set up the command blocks in your minecraft world like the picture shows Then add a 4 repeater clock (2 repeaters on each side) and set the tick delay to 3.
So in the left command block (attachted to the clock) put in 
/execute @e[type=Snowball] ~ ~ ~ summon PrimedTnt
and then in the command block attatched to the repeater put
/kill @e[type=Snowball] 
So what this does wherever there is a snowball it will blow up when the command block is activated that's why we want a slow clock.

     
      Or if you want to use an extra command block (so tough) You can have a normal clock the fastest one possible do
/testfor @e[type=Snowball] then after about 5 or 6 ticks however many repeaters you want to delay then do the  execute above. So let's say you use 5 repeaters on maximim delay. So that is 2.0 seconds. So after the snowball is alive for 2 seconds it will explode and then have a command block right next to it doing /kill @e[type=Arrow]