You've gotten some great advice.
And you've gotten some questionable advice... While I truly think her intentions are good, Jackie B's advice about feeding on the mom's schedule every 3-4 hours (instead of following and trusting your baby's cues) will NOT help you increase your supply at all. It will likely make your baby fussier than necessary and probably will harm your supply even more (due to decreased frequency of your baby at the breast). First, the myth of "a baby can't be hungry 20 minutes after they've nursed." Babies *can* nurse and then be hungry again within 15-20 minutes - this is TYPICAL behavior during growth spurts!!! This is how babies are *biologically programmed* to increase your supply! Disrupting this w/ scheduled feedings only every 3-4 hours is only going to harm your supply. Doing scheduled feedings when mom prefers (instead of when baby prefers) can actually be *more* tiring than feeding on cue because baby is likely more fussy if he's hungry but having to wait around for the next scheduled feeding (shoot, I get grumpy when I'm hungry but can't find food, too!, and I'm an adult!).
In order to maintain & increase your supply at this age, your 3 month old NEEDS to eat *on cue* - meaning *whenever* he's showing ANY signs of hunger (sucking on hands, rooting - crying means he's already really hungry!). Also, your 3 month old should be nursing at least 2x during the night, too - even if you have great supply, he's just too young to not nurse at night, but ESPECIALLY since you're experiencing low supply, the only way to increase supply is to nurse frequently, day AND night.
Now, on to the excellent advice - just mostly summarizing here from what other moms have said plus a few links:
* http://www.kellymom.com/bf/supply/low-supply.html
* lots of skin to skin w/ your baby (nap together if you can!, wear him in a sling close to you, take a bath together, etc.
* feed on cue - TRUST YOUR BABY to tell you when he NEEDS TO NURSE, when he's hungry, and that *could be* every 20 minutes at times at this age!!! Young babies often do these frequent nursings (called "cluster feeding") during growth spurts to specifically increase mom's supply to keep up with their nutritional needs... pretty cool biological design, eh! ;)
* for now, no artificial nipples or pacifiers (at least until your supply is sufficient) - even light suckling at the breast for comfort will stimulate your milk supply to increase
* instead of supplementing with formula, PUMP PUMP PUMP w/ a double electric breast pump (10 minutes on, 10 minutes off, 10 minutes on, 10 minutes off) and supplement with your own breastmilk
* eat & drink natural galatagogues (things that increase your supply) like: oats/oatmeal every morning/oatmeal cookies/oatmilk, eat as much garlic as you can stand, take fenugreek capsules or tinctures (like Mother's Milk Tincture, etc.)
http://www.drjaygordon.com/development/bf/galact.asp
* avoid peppermint & sage & decongestant medicines (these can decrease supply)
* please, please attend a local LLL meeting, call a local LLL leader, and get a recommendation for a good, well-respected lactation consultant (IBCLC) who can help you get off the formula and instead increase your supply - every time you use formula, you are hurting your supply - even if you pump, it is not as effective as your baby's sucking
LLL of Dublin: Meets on the first Monday of each month at 9:30 a.m. at the Dublin Presbyterian Church, 5775 Dublinshire Drive. Please Tiffany 614/477-2970 for information.
http://www.discoverohio.net/LLL/index.shtml
* with the help of LLL and/or an IBCLC, mentally prepare to *ditch the formula* because it's not helping your supply at all - this is why formula can be such a slippery slope! Yes, sometimes formula can help moms "get over a hump" as long as it is used incredibly sparingly AND the mom is *pumping* both breasts with a very good double electric pump EACH TIME formula is given - but, for the most part, when moms use formula, it can so easily become a crutch where there's nothing being done to even maintain (let alone increase) the mom's supply... so you need to work to increase your baby's time at the breast, increase your pumping, and keep the goal in mind to get off the formula (whether that has to be done gradually or could be done cold turkey, you'd want to work together with someone who is supportive of and specializes in breastfeeding, like a lactation consultant or LLL leader).
* stay well hydrated (lots of water!) and well fed, and REST as much as you can! For now, look at "feeding your baby at the breast" as your *primary* job - increasing your supply now will benefit you & your baby in SO many ways in the months to come and is really a great investment of time and effort!
So, CONGRATULATIONS on the birth of your son, on being able to stay at home to watch him grow, and choosing to feed him at the breast w/ the amazing love & health benefits that breastfeeding brings. You're doing wonderful work that can be tiring at times at the beginning, but HANG IN THERE - you're in the learning stages right now, but this will soon become 2nd nature and will likely become extremely easy & convenient once your supply is up to where it needs to be. Best of luck to you both - you CAN get your supply up w/ correct information, support, and determination! :)