Well, HPV is so slow to cause serious changes, that even if she had a pap smear, then a new partner immediately, three years later really shouldn't be an issue despite the new partner. You're still going to catch any abnormalities far in advance of them becoming a serious issue. Especially in the US where pap smears are done so overly-frequently anyway, I imagine if you told your doctor that you had a new partner and would prefer to be tested every other year or even every year instead of every three, they'd probably explain the logic behind the recommended schedule but still ultimately let you determine when to get tested.