Hi Eros
You can first invite your girl to come to Belgium for a sort of trial period of three months. My friend did this some time ago (also Belgium). But the Thai Embassy in Brussels can not make this happen. Your girl will have to go to the Belgean Embassy in BKK to maker it all happen.
But first you need to sent her some papers: go to the city hall in the city you live in and ask for a special document (which is a sort of proof that you have a house where you can stay, an adress). As well you need to sent her a recent copy of a pay check, as a proof you have an income. You also need to sent her a motivation letter in dutch or English. Make sure you sent original documents not copies because this could create problems and will slow up everything. The only copies you need to add are your id-card and a copy of her id-card passport. Also good thing is to add a picture of the both of you together. Do not sent it with normal post, but with express post. Go to the post-office and ask about the different ways you can sent it to her (because normal post takes very long to arrive in Phuket sometimes).
Your girl has to take all this documents to the Belgian Embassy in BKK and fill out a sort of request form. Also proof that you now her allready 3 years might help a lot (phone bills, e-mails, flight tickets....).
Then you will have to wait and see what happens. Also tell your girl that she makes sure she keeps the request-number with her and that she tells you. With this reference-number you can look up the status of the request on the
www.fgov.be (hope this is the right website). Or you can allways call the Thai Embassy in Brussels for a status report (but I've been told if you call to much the will get fed up with you.)
Even if you think you are really sure about this girl, start with a trial period of three months first. Maybe she will get homesick or something like this, only future can tell.
My friends girlfriend (she has no kids) was here for three months (during summertime 2003), then she had to go back because the 3month visa expired and now since (middle december 2003) she's back in Belgium and she can stay for a period of 5years...after this 5years my friend has to decide about marriage or not. Until now it's going fine. But once again think about it and talk talk talk, with her, your friends and your family before you put a lot of energy in all this.
Please realize that saying goodbye after 3 months is much more difficult than saying goodbye after 2 weeks Patong.
Once again I just wanted to provide the info you asked for, but it's all up to you. To be honest i'am quite critical about all this, I mean I would never do it myself, but it's your decision and i do not know your girl.