Paypal's growth in popularity is probably due to its convenience and ease of use, its buyer protection, and (believe it or not) its reasonable price for the aforementioned. Apparently people will put up with a surprising amount of whimsical tyranny if it means they don't have to go purchase a money order and
hope it gets to an honest seller.
Paypal charges a $.30 transaction fee plus 2.9% of the amount sent. If you already know the amount you're charging, then it's easy to calculate how much the fee will be as well. (Or you can just go to a site like thefeecalculator.com if you are math-averse.) Per your example, Paypal would charge $.39 for a $3.00 payment, leaving you with $2.61 of actual money.
However, Paypal doesn't always charge you a fee for accepting payment. If your buyer pays from an existing fund like a Paypal balance, then there shouldn't be a fee. Or your buyer can pay with a credit card and choose the "gift" option and pay the fees themselves, but this isn't always a good idea since you obviously can't dispute a "gift" if something goes wrong.
These are just the basic permutations, but they'll probably cover about 80% of your trading in my experience. The fee schedule seems to get a little more arcane for payments of non-U.S. origin, and sometimes you'll find that something you thought wouldn't generate a fee does, and vice versa.
Some people think it's very bad form to ask the buyer to foot the bill for the fees (unless you're specifically offering something at cost/as a favor, e.g. Fair ponies, freebies) and some people think it's literally a small price to pay. If you insist on passing the Paypal buck to your customers, the best thing is to make sure you're either up front with your policy, or just bake it in as part of the pony's cost. Want $3 for that pony? Make it $3.50 and your potential fees would be covered, plus you don't risk insulting your buyer. Otherwise you just have to suck it up and consider it a cost of doing business conveniently.