@AB21 the following checks whether or not the room is booked for that particular 1 hour time slot. I didn't understand what you meant by more than 3 people booking a room, as surely you only want 1 person to book it and everyone else to get the "sorry" email.
Form the form is shown below and because Forms doesn't have a date/time field we have to ask for the time separately. To make it easier it is best to have specific 1 hour time slots (or 30 mins, 2 hours ....)

List
Our SharePoint list has the Title column for the booker's name, Room is a Choice column and Booking is Date/Time.

Flow
As always with Forms, start with the when a new response is submitted trigger and the get response details action. I've added the Office 365 Users get user profile (v2) to get the first and surname of the responder.

Next, we add a few compose actions to get the room, date and time requested.

We then use a compose to concatenate the date and time. I was having a bit of trouble with time zone conversions so added a couple of convert time zone actions to make sure the list and filter query were in step. And to make sure the email had the booking date in the correct format I added another compose for that.


Next, add a get items action and in the filter query check for the room and the booking date/time.

Next we need to check how many items were returned from the get items action. So add a condition with length(outputs('Get_items')?['body/value']) is equal to 0

In the green if yes channel - that is, there are no clashes - add a SharePoint create item action and the send an email action

The email that will be sent looks like this:

In the red if no channel add the send email action and it will wrap itself in an apply to each which is expected.

And the email looks like this:

I hope that gives you enough to work on.
Rob
Los Gallardos
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