Welcome to parenthood, and specifically mommy-hood!
It could take me pages and pages to answer your question, so I will just let you know that you are completely normal to have a low or no sex drive with a new baby. Many women do not experience desire for their partners or any type of physical intimacy for many months...even a year... for so many reasons:
1. "Touched-out". Whether you are breastfeeding or not, you are physically near another human being almost constantly, especially at the newborn stage (and, well, then at their separation anxiety stage and teething stage...). Women feel "touched out" meaning they have been physically touched all day by their sweet and/or crying baby, and the last thing they want to do is be touched again (even by their sweet and sexy partner).
2. Overwhelmed. This speaks for itself, but being a new mom is exhausting, draining, overwhelming for so many reasons. These feelings don't tend to lend themselves to feeling sexy and desirable!
3. Hormones. The first three months of a newborn's life is known as the mom's "fourth trimester', as the woman is going through enormous changes with her body's healing from childbirth (I am assuming you did not adopt your 3 month old), possibly breastfeeding and hormone levels changing. Your body went through miraculous changes for the last 9 months...everything from your blood volume increasing by 50%, to your pelvic muscles expanding...and all of that must go back to its (somewhat) original shape and location in your body. Your hormones are no exception!
4. Evolution. There are theories that women purposefully do not have a "sex drive" in the first few months (or more) with a newborn or a baby, as they need to conserve their energy for the baby, and prevent another pregnancy too close to childbirth to have a healthy next pregnancy.
There are a million other reasons or possibilities, including if you are feeling sad or depressed, isolated, struggling with a new identity or change in lifestyle, dealing with a partner who is not helpful or unsure how to help...the list goes on.
Please understand that it is OK not not have a sex drive right now, and you can be intimate with your partner in other ways. I am unsure from your question if you are worried or if your partner is the one concerned about the lack of sex...but please find ways to be intimate, whether that may be emotional intimacy right now.