No, neither are right, you should only have 1 answer as well. The number of photons should definitely be greater than 1.

What did you do?

Use E=m*C*delta T to find the Energy that is needed to raise the 5 gallons of water by 3 degrees.

(For m, convert 5 gallons to grams)

Number of photons needed= this value/energy of the photons (see my last post)

I think confusion lies with Specific Heat...

C=specific heat=4.184. This means that it takes 4.184J of energy to raise the temp of 1g of water 1 degree. For every gram of water you need 4.184J to raise it's temp by 1 degree, (4.184*2)J to raise its temp by 2 degrees and so on.