при получении гостевой визы 3bis обязательно?

По идее вам должны дать визу без 3бис исходя из информации на diplomatie.be:

4. Documents proving that you have sufficient means of subsistence, covering both the duration of your stay and your return journey:

either your own financial means (e.g. hotel reservation, cheques and credit cards accepted in Belgium, an employment contract, bank statements, proof of enrolment on the trade register and/or of professional activity);

or the guarantor's financial means: A pledge of financial support is specific proof of means of subsistence. It offers a solution in situations where you cannot prove your own solvency. Through this pledge of financial support, a Belgian national, or a foreign national resident in Belgium, acts as a guarantor for your period of residence, your return journey and your medical costs. The guarantor should ask for the pledge of financial support (also called Annex 3bis) at the municipality (commune/gemeente) of his/her place of residence. The guarantor does not have to be the issuer of the invitation. When the pledge of financial support has been authenticated by the municipal authorities, the original document must be submitted to the relevant embassy or consulate, within six months of the authentication, together with:

a copy of the guarantor's last three pay slips or any other (official) document proving his/her solvency.

a copy of a document proving that the guarantor is of Belgian nationality (identity card) or has authorisation to stay in Belgium indefinitely (residence permit).
Given that the number of current dependents is a factor in determining the solvency of the guarantor, it is also advisable to submit proof of the make-up of the guarantor's family together with evidence of any family benefits received.

Но при этом вы должны иметь-таки 3. Documents proving the purpose of your trip (e.g. a letter of invitation) and the circumstances of the planned stay (e.g. hotel reservation, staying with a private individual).

Удачи!