Create unique field counts from comma deliminated line

[chablist]

I have a log file that always has the same structure of:
time1,time2,groupNumber

eg:
355350224,338837556,2
1355350228,338837557,2
1355312572,338828143,4
1355350212,338837553,4
1355350216,338837554,4
1355350220,338837555,4
1355350224,338837556,4
1355310304,338827576,5

I want to perform a search such that i can get a count for each of the unique groupNumbers for a report to be able to return something like:

2: 2
4: 5
5: 1

[sowings]

Sounds like the distinct_count function of stats. Or perhaps just count:

<your search here> | stats count by groupNumber