If you have administration level access, go to the home page and click on users. Once in the user's view, click yje button to get to the advanced sorting tools. You want the Last login to be checked and set the "is after ..." set to a date three months ago. The system will not give you a numberical count, but by counting how many there are on one page you can multiply by the number of pages to get a pretty good estimate.
Ours had 30 per page and we had 38 full pages for a total of 1140 unique log ins since our school year began in the fall.
Another option you may enjoy is embedding a counter on to your home page. I like FlagCounter. It will post your unique visitors, page views, and also list all the flags from the countries that your students have logged in from. We have had ours on the site only one month but we have had the fun of seeing that 41 countries worth of flags are showing up. Another advantage is that you can compare the FlagCounter's page views to your log-ins that Moodle shows to get an idea of how many actual visitors, not just students, have hit your Moodle home page. Our counter shows about 4,000 total unique visits. That tels me that 4,000 minus our Moodle;s reported 1140 student log ins gives about 2800 hits that are not students. That would be the 'curious about your program', the web spiders, and the people just wanting to see what look your Moodle has to give them ideas about how they might design theirs. :0)