I also have a 3 year old boy! We bought our 1 year old a pop up play tent, some books and bath toys. They already have a ball pit with ramp and steps etc, and outside we have a little trampoline, mud kitchen, slide, sand and water table so hopefully they will enjoy playing with all of that soon
Maybe make their present a day out to the zoo or aquarium I'd you don't need more things.
I’m thinking “out doors”, my girls love the play tent at soft play so I’ve got them one of them with a tunnel and ball pit attached. I’ve also got them an outdoor activity table you can put sand/water in and to use for messy play, both thing for less than £15 each. Also going to get a good paddling pool. But I must say, they’d be happy with absolutely anything. So easily pleased at this age so I’m taking advantage of that!