How to do permutations and combinations?

Question

I have a scenario like this:

Three items with priority 0. 

Let that be priority_0_1, priority_0_2, priority_0_3

Two Items with priority 1. 

Let that be priority_1_1, priority_1_2

One Item with priority 1. 

Let that be priority_2_1

So i want all the outputs as follows:

priority_0_1, priority_1_1, priority_2_1

priority_0_1, priority_1_2, priority_2_1

priority_0_2, priority_1_1, priority_2_1

priority_0_2, priority_1_2, priority_2_1

priority_0_3, priority_1_1, priority_2_1

priority_0_3, priority_1_2, priority_2_1

Note: The above stated is just an example. 

Number of priorities and the number of items in a priority will vary.