We aren't entirely clear on your question, but I will answer this line:

#### Lyndamurphy wrote:

I have a monthly total and want to estimate over 3 months

If you have just one month and want to estimate for 3 months, the easiest way is to multiply by 3.

Example: April=60, so April, May and June together can be estimated to be 60×3=180

More accurately you would consider the number of days in each month. Since April has 30, May has 31, and June has 30, you would calculate as follows: Daily Average in April = 60/30 = 2, so estimate for the three months = 2×(30+31+30) = 182