18mm chipwood boards over 2 by 6 joists

Because the joists are spaced 600mm centers it is always best to use thicker chipboards unless you can use plywood which has a better density

I think a 6 x2 would struggle to support the extra weight at 600mm centres and 3m span I would add extra support and use 22mm chipboard

All depends on the structural grade of the 6x2 joists and what they are loading on. If they are graded, they should have a stamp printed/marked c16 or c24, highest grade would suggest a greater capacity to bear weight. If there are no markings then typically I would suggest strengthening the floor with noggins of the same depth or just shy in each bay or doubling up the joists lengthways to reduce centres and use 22mm chipboard to keep the additional weight to a minimum, also spread your storage evenly. If the joists are marked c24 then an 18mm structural ply would suffice without strengthening the floor. If you do decide to use chipboard without additional strengthening, I would strongly advise no less than 22mm.

Boarding up only for storage would be OK. Anything else you need structural calculations. Going for thicker boards will add weight to the joist as well. So 18MM OK.