Suppose you talk to your girl one day, and you decided to have a date two days from now. Later that day, you browse the net, and order a book, and it will be delivered 3 days from now. Then you go to a tailor, and order a new suit, and that will be done a week from now.

Now, let's say all this happens in a game.

Assuming the player and/or character is willing to do all three of those things, is not in any particular hurry, and has enough money, what sort of in-game reason would you give for forcing player to choose only one of those three. The sort of realistic reason that would actually make common sense, not just "picked 0 out of 1". And more, even though the player/character is not really in any particular hurry, what sort of reason would you create to make player care about how much in-game time will each choice take to complete ?

This example seems rather contrived - and finding reasonable excuses for contrived problems might be difficult.
Is there any reason such a problem must be presented to the player? Why shouldn't he have all three if he had the money for it?

Forcing the player to choose one:
You could put them all at the same point in time, so choosing one is forced by only being in one place at one time.
Or, you could limit his money.
Or, his smartphone battery could be running low and the player knows he only has enough charge left to A) Call girlfriend, B) order book from website, or C) Make an appointment with the tailor. Battery blinking on it's last bar: [---=]
Or the same thing, but his car is running out of fuel to visit girlfriend to ask her out, or visit the bookstore, or visit the tailor.
Or, the same thing but the time left in the day is running out, and the bookstore and tailor would be closing soon, and the girlfriend would be asleep soon.

Forcing the player to care about the amount of time:
The choices all take place in Town X. In Town Y, something is happening continuously, and the longer the player stays away, he either suffers (whatever is happening in Town Y gets worse) or misses benefits (would've gained by being in Town Y).
Or, the choices all take place in Town X, but Town X is going to get attacked soon, only you don't know exactly when, so staying in Town X longer increases your perceived risk of being attacked.

The very essence of a game is the concept of artificially restricted choices, so don't overthink it too much. You can literally just present it as a 3-way choice if you like. Servant of the Lord gave some great suggestions for how you can cover it up with narrative, but you may not need to.

Is there any reason such a problem must be presented to the player?

Well, naturally. Let's say that you have an option presented to the player before the "level", that will somewhat help to cope with the regular gameplay. And you have some other options present in a list as well. Well, why shouldnt the player just pick every one of them ? And if there is an option that the player will pick every single time - why is it in the list in the first place ? It makes sense to just automate it. But i dont want to. I have just such an option, that a player pretty much should always pick before playing a "level", except what i want, is to make it a conscious choice on their part. It is desirable, yet it must be considered if some other options are vying to be attended to, and it has to make sense as the game itself is relying rather heavily on common sense.

You could put them all at the same point in time, so choosing one is forced by only being in one place at one time.

Well, except, as in my example, all options are rather prolonged in time, and there is actually no apparent reason (if it were in fact happening in real world) why you cant do them simultaneously. Same with the other solutions you provided. Since all the choices take place over several days, nothing really stops one from charging the phone/refueling the car.

Or, you could limit his money.

It would work, of course, but the problem is that one of the end-game goals is to amass as much money as possible. So if the player decides to go for that ending - the solution stops working entirely.

the choices all take place in Town X, but Town X is going to get attacked soon, only you don't know exactly when, so staying in Town X longer increases your perceived risk of being attacked.

This one would solve the limited time part rather nicely.

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Or, you could limit his money.

It would work, of course, but the problem is that one of the end-game goals is to amass as much money as possible. So if the player decides to go for that ending - the solution stops working entirely.

Could, then, this not be a valid point still? Such as, you are limited in what you can do at one time unless you decide to amass a fortune. In that case, like in real life you can go out more, buy stuff, etc.