Yes this is wrong. You are not a random girl that he has been dating. 3 years is more than enough time for them to have included you in a birthday dinner. I would talk with your boyfriend as he knows his family more than you would as to the snub. Was it assumed you would be there therefore no formal invite took place? When I was dating my husband they just told me where it was going to be since I was a fixture in his life it wasn’t a discussion of if but when.
How is the relationship with the family? What underlying issues are happening that you are aware of? Sometimes we not there is salt in a wound and if given an oppportunity would snub them just the same. If that is the case and you want to be included than you have to sit down and have a talk with whomever is the matriarch or patriarch of the family and iron out some differences. However if your boyfriend has any sense than he has already spoken on your behalf. My personal rule is to always allow the person whose family it is to talk it out at first. I am hoping it was just an assumption. How did you hear about the party? This is key too. If his mom told you but sent no formal invite by mail then its safe to say you just need to go and celebrate your man. Is it a surprise? Then speak again to whomever is throwing the party and go from there. Sometimes taking the first step in communication will be beneficial.