2 girls and 1 guy sharing a hotel room

My friends and I are planning a trip to Japan this Spring and there is going to be on odd number of us going (5) 3 girls and 2 guys. 1 girls and guy are a couple and will share here own room but the rest of us were thinking of sharing a room and were wondering is this allowed or looked down upon?
We want to make sure that we won't run into any trouble before we go.

the rest of us were thinking of sharing a room and were wondering is this allowed or looked down upon?

Generally it will not be a problem, assuming you are able to reserve a room meant for 3 people (which is not as common as you might think). However you may find it easier to find accommodation if you split up into two doubles and a single. Also, there are a handful of hostels that restrict non-married couples from sharing rooms, but they are extremely rare.

I think one guy and two girls in a room will be fine, but if you each want your own bed bear in mind that there aren't a whole lot of options for triple rooms, a number of hotels will not have any.

You don't mention a budget but if you plan on staying in relatively cheap business hotels you may find that a single room and a twin room for the three of you will be a better option and in some instances three single rooms might even be better. In general Japanese business hotels charge per person not per room, and a lot of them do not have triples.

It really depends on where you are going and where you plan on staying, at what price. If you want some suggestions feel free to post your budget and itinerary.

Enjoy your trip!

This is very commonly done, not as much at hotels where rooms are smaller, but at minshuku.

Minshuku are Japanese-style usually family-run inns where you are expected to carry the dishes to your table and fold up your futon bedding.

Minshuku is practically a huge house, and they can provide a huge Japanese-style tatami room upon request where you will be free to lay your futon at any place you want.

But usually there is no lock on your door. You have to trust the owners for security.

Maybe worth considering.

Thank you for all the suggestions, very helpful. As for trip schedule we start in Tokyo, Yamanouchi, Sendai, Nikko, Nagoya, Osaka, Kyoto, Hiroshima then back to Tokyo. As far as a budget about a medium level 10,000-20,000 Yen per person with a splurge here and there.

I am a girl, and on one trip to Japan I travelled with two friends that are guys. We shared rooms whenever we stayed in ryokan and minshuku. It was never a problem, though a couple of times the owner would suggest to set up some sort of room divider, which we said no to as we thought it would be unnecessary trouble. On both these occasions, the owners did look slightly shocked or at least very surprised, LOL.

U can ask a twin room with extra bed.
Then no one think , U3 make any affair.

I think at least a few branches of the Villa Fontaine hotel in Tokyo offer triple rooms. But if you're doing it just to save money, a single and a twin are often close to the same price, and you get two bathrooms.