Arthur received his green card after being sponsored by his employer. He also married a US citizen around the time his application was being processed and they have been married for more than 3 years and he now also has his green card for more than three years. He asks, "If I apply for naturalization, will the USCIS question the validity of my marriage and start a new investigation the way they do when granting permanent residence? My marriage is perfectly ordinary and I have nothing to fear but I wonder if I will be questioned about the color of her toothbrush."
The strictest investigation happens when an immigrant receives a green card through marriage because the USCIS wants to make sure that there was no sham marriage and no fraud was involved. For naturalization purposes in your case, since you are taking advantage of the 3-year early option, the USCIS is not going to be as probing as it is in the case of marriage based green cards because it is clear that if you wait 2 more years, your marriage to a US citizen becomes irrelevant.
You must provide satisfactory evidence of your marriage in form of a certificate and joint taxes. If possible adding a copy of joint bank accounts and/or mortgage statements will make the case even more convincing. If these things are in order, then, you can expect only confirmatory questions related to your marriage and spouse (are you still married? do you both live together?) but unless the officer suspects something, you will not be asked anything more.