Given a list containing monthly numerical data, how can I easily convert this into quarterly data?

```
x= [5,8,3,4,5,6,1,2,5,3,11,8] #monthly data for Jan-Dec

```

Desired output: 

```
[5+8+3, 4+5+6, 1+2+5, 3+11+8] #converted to quarterly data

```

I wanted to do something like `[a+b+c for a,b,c in x]` but x it says x is not iterable.

I don't think this is a duplicate. I am specifically looking for a list comprehension solution.