How many full-time volunteers are donating their time in the US, if you add up the average 3 hours that the 100 million volunteers are donating?

7.5 million equivalent full-time volunteers

Explanation:

The total hours volunteered is 3 hours * 100 million volunteers = 300 million hours

If you assume a full-time work week of 40 hours, divide the hours volunteered to find the equivalent number of full time workers.

300 million / 40 = 7.5 million