If I were asked to choose only three differences--and they had to reflect the daily experience of men and women--then I'd go with the following with one caveat: that I couldn't keep it to just three. Four was the best I could do.
1. Concern about Sexual Assault: When a man walks down an empty hallway or up a quiet street, he seldom needs to worry about being raped. How often does a woman do the same without concern about rape somewhere in the not-so-deep recesses of her mind?
My wife puts it another way: If a naked man were to walk into a room full of women, the chances are he won't need to worry about being sexually assaulted. If a naked woman were to walk into a room full of men, she would be very lucky if she weren't sexually assaulted.
2. Being Able To Pee Standing Up: If I as a guy had to sit down every time I peed, I'd seriously consider shooting myself. If a woman were to inhabit a man's body for a week, I do believe this would be the thing she missed the most.
3. Concern about Pregnancy and Motherhood: It's not like we guys aren't on the hook financially for the children we spawn, but women are way more stuck in the child-rearing arena than men often are. Double that for being able to enjoy sex without concern about unwanted pregnancy.
This isn't to say that women are better parents or that there aren't plenty of men who take total responsibility for the children they conceived--and some who they didn't. But the burden of mothering is culturally expected of women, while being a devoted father is not.
4. Periods. While menstruation isn't a part of a woman's daily experience, breakthrough bleeding certainly can be. How many guys need to be tethered to a tampon from the ages of 11 to 50? Sure, adolescent males need to worry about the occasional unwanted boner, but how many males have to worry about standing up and having a friend rush up to tell them there's a big spot of blood on the back of their pants? How many guys need to deal with cramps? And if you if think that period suppression helps even the playing field, then you don't know much about having to put artificial hormones into your body.
These are my three--uh, four. What about yours?