Early mortgage repayment

Honestly, why ask this question here? Just call Belfius, they will tell you and give you an accurate and definitive answer based on the type of mortgage you have, the term etc.

At least 3 month's interest, but I doubt that helps either.

Usually you pay 3 months interest.
So on a very rough simulation :

Interest : 2%
Mortgage repayment : 100,000

Interest 2% / 12 = 0.16666% (Monthly Interest)
100,000 x 0.16666% * 3 months = 499.98 EUR penalty costs

Other items might come into factor like the date when you pay the capital. Daily interest might be added from the last monthly installment to the day when the capital is fully repaid. Or other conditions that are written on your mortgage and we might not be aware.

However Belfius is usually pretty fast in responding and you can have your answer in a couple of days max.

How much of a fee/ penalty you would pay will depend on a wide variety of factors and could be from nothing to a few months interest.

For example, if your interest rate is fixed for the next 10 years, then the penalties will likely be higher than if you have a variable market rate.

A 30 year mortgage will have a higher penalty than a 5 year mortgage. etc. etc. etc.

As others have said above, no one here really has any way of knowing for sure as no one here knows the terms and conditions of your mortgage.

The only way to know for sure is to call Belfuis and ask them.

You could also ask your notaire, who will have to contact Belfius anyway to get the amount as he/she has to pay them the correct amount from the proceeds of the sale. (They also have to deduct things like unpaid précompte immobilier, or estate agent fees, etc. etc. if those things apply)

Forgot to mention that you most likely have to pay to remove the "hypoteque" on top of your property, which is a sort of a guarantee the bank usually put on top of the apartment for the whole length of the mortgage.
Once the ownership passes to the next buyer this hypoteque needs to be removed at a cost (the Notaire calculates that).

If your interest rate is fixed for the next years, then the penalties will likely be higher than if you have a variable market rate Lite Blue.

The way that you word the question suggests that you don't fully understand how this works.
'I……….will use the capital to pay off my mortgage early.'
No. The notaris who deals with the sale will hand over the sale money to your mortgage lender (who has the first claim) and you will get whatever is left after the remaining loan and all the charges and legal fees are deducted.
You have no say in the matter.