2-3 Bedroom Furnished Accomodation Needed

Short or long term? Unless you have plenty of times to do reccies in advance and choose long term accommodation, short term furnished is a good idea. That way you get more time to look for long term, get to know areas, practicalities of where to live in relation to any new home.
For short term, look at Home in Brussels website, you'll find 1 bed flats at very good prices for under 1k per month.
For long term, look on immoweb.be
Long term UNFURNISHED is far better than furnished  - you damage any furniture and you will have to pay replacement costs, even if it is not replaced, at the end of your tenancy. Furnished nearly always costs more too and there is less of it, as landlords pay tax on furnished accommodation but not on unfurnished. Where you live will be dictated by where you are working and budget. However, even in the most desirable parts of Brussels, a 3 bed house can be found for 1k per month, less desirable you could find for 700 per month. Avoid relocation agents and do your own homework. Almost NEVER accept a fixed contract, well 1 year might be ok, but unless you have it written into a fixed contract a "get-out" clause, you will pay the full duration if you leave early. 9 year standard contract is a far better idea.