I did that with a plastic drum about 6-7 years ago. Did the same thing for the lower links with cat1 3PT pins, but for the top link I got a used anti way bar, cut it in half and had the two ends with holes sticking out the front side of the barrel near the top to attach the top link to. Worked just like any other 3PT implement.
Funny thing is, I filled it near full of cement. I thought it would weigh quite a bit, but I could tip it over or stand it up by hand. I estimate it weighs less than 500 pounds. Be sure to put the lower pins in high enough so that your 3PT lift arms can get down to it. Sounds obvious, but I put mine in near the lower end of the range of motion, and let the barrel sit for a year or two. It sank into the sand down past the limit of motion of the arms. So I had to knock it over, roll it a few feet and stand it up on soil that was a few inches higher. Must have been a design flaw /forums/images/graemlins/grin.gif
Also, the plastic barrel deformed a bit as the concrete cured. It looks kind of lumpy, but it is functional. You could probably solve this by doing it in a few pours, not one big lump. I don't know how this would affect the structural integrity, so use lots of re-bar. The next time I go out to my property I'll try to rememer to take a picture of it.