I'm building a 3kg LEGO EV3 sumo robot and I need to make it as small as possible.
Which LEGO items do you recommend to add more weight on it? (tires, plates, wheels, blocks, minifigs etc.)
The 2 x 6 x 2 weight elements are pretty good.
Train Magnets are also pretty dense, though not very large and are now rather pricey.
Any solid rubber wheels also pretty dense. Avoid the hollow ones full or air.
If you need a lot of weight, you might also add a battery box, full of batteries of course. These are the ones currently in productions, but older ones will work just as well.
I've also seen people add AA batteries without a battery box, but that may be bending the rules a bit.
Most LEGO Electronics are actually fairly heavy. You could also add motors, old LEGO RC stuff, or even whole other Mindstorms or other robotics bricks.
Make sure to check the rules for your sumo competition to see if any of these are excluded.
A second EV3 with a battery pack is one of the best things you can add to the existing design, provided your robot has space to conveniently house it (but sumo robots typically do). It is actually dual-use: not only does it give weight, but you can add even more motors or sensors to your robot design.
There's an added bonus if you use the EV3 with the battery pack (yeah, I know it's not exactly cheap) rather than ordinary batteries, though there's a small change in the space occupied by the brick.
Another thing in terms of weight is the motors: if you're only using, say, 2 EV3 regulated motors, consider adding another two right next to them via gears. This gives you the 2x weight you want + doubles the torque.
Yet another things I can advise - and you've got to check the rules of your particular competition - is to use axles made of metal. The reason I'd suggest this is that although "it's not LEGO" in the traditional sense, LEGO Technic axles are very weak and don't allow you to hang a wheel 'loosely' off a bar. With metal axles this problem is removed somewhat; plus, if you're in control of axle length, you can actually make very long axles that go through the entire robot, providing extra rigidity as they become 'load bearing' for wheels on both sides. And yes, this also gives you the extra weight.
Finally there's the extreme option of just having heavier parts, again if and only if this is permitted by the competition rules. The two things I can think of are:
Speaking from experience, you'll find that adding weight doesn't really help in Sumo competitions as much as you want. One reason is so many robots these days have front-facing scoops that try to lift you up, and I'm yet to see this approach fail when the robot is too heavy.
You could add an EV3/NXT large motor. They're not the most compact things but if you have some spare size you could easily add one of these.
You can add sensors. They don't need to be plugged in. I use a lot of touch sensors and motors.