C.,
Although I cannot personally report a problem with my voice during pregnancy (my singing voice is never worth listening to!) I have definitely heard that the hormonal changes we go through during pregnancy can affect voice quality. I'm thinking I read somewhere that hormones can change the shape of our voicebox, thus changing our sound? In addition, the baby pushing our organs out of place can effect our ability to get a good breath for singing.
I don't think that there is anything you can do about it. Sounds like your body has not yet recovered from birthing 3 babies in such a short period of time. If you are still nursing that could prolong it. I do believe that it goes away (or your voice comes back), so give yourself some time. It can take at least four years from the time we stop nursing for our bodies to get back to normal (I'm not talking about weight, but hormones).
Hopefully someone will have more scientific details than what I have shared, but I did want you to know that there is nothing wrong with you and it should end in the future!
D.
mom to 5 children