It means 3 plates on a barbell. Which actually means 6 plates, 3 either side. A plate is 45lb while the bar itself is 44lb.
Put them together and that is a huge amount. Considering the usual 1/2/3/4 standard, he is talking about the bench press exercise.
Here's what it looks like:https://www.youtube.com/watch?v=Ji7Wl-KV-aM
(warning: not safe for work)