A forty is a 40-ounce bottle of malt liquor. Malt liquor, it needs to be noted, is at its simplest, strong beer that has been made stronger by the use of adjuncts such as sugar or corn that help bump the alcohol percentage up a few ticks. In other words, it's not liquor in the sense that rum or vodka are liquors.
A drink is 1 beer or 1 shot of 80-proof liquor. You have three shots of liquor with half the usual alcohol content, so that's like 1.5 drinks. 1.5 beers would have the same amount of alcohol.