Uhmm~That's a good question.
I am sure that all of them do have a meaning of 'Where...' as you said. Generally speaking, they can swap with each other in daily life (and this always happen), which make it easy for me to explain all these three at the same time. Yeah there are no big differences BUT small ones.
In my mind, those sentences are not clear if you just give me them without any situations. “这” is a pronoun. It can refer to many many things, which makes these sentences have wild meanings.
Their relationship:
“这是在哪儿”＝“这是在哪”＞“这是哪”
————————————
Examples here~
1. I come to a place that I don't know so I am really confused, or I only know a name of someplace. I may ask “这是（在）哪（儿）”. “这” can only refer to the PLACE. And I need more details of this place. The details includes the location and the environment...Just anything related to the place. We often use “这是哪” in such a situation.
-（开车开到了荒地。）
-( Drive to nowhere.)
-这是（在）哪（儿）？
-(Where am I ???/ What is this place ???)
-汤姆去M国了。
-Tom went to M country.
-这是（在）哪（儿）？
-Where/What is 'M country'?
2. When “在” appears, the question is not only able to ask for the 'details of some places', but also the location of obejects/people/cases... “这” can refer to anthing we like.
-珍妮打架了。
-Jenny got into a fight.
-这是在哪（儿）？
-Where did the fight take place?
-看这张海滩的照片。
-Look at this picture of a beach.
-这是在哪（儿）？
-Where is the beach?
-我找不到我的帽子了, 这是在哪（儿）？
-I can't find my hat. Where is it?
-我找了，给。
-I find it! Here you are.
-谢谢。这是在哪（儿）？
-Thanks. Where was it?/Where did you find it?
3.“这是在哪”and“这是在哪儿” don't have differences in meaning. “儿” doesn't have meaning in the case that it is a kind of accent or talking habit.If you want, you can add “儿” after every word XDD. That's not a big deal. You may choose where to add or not to. Remember to say it softly with no tone, or other people may misunderstand “儿” as “儿子(son)”、“女儿(daughter)”、“孩儿(child/children)”.
Habit & Accent:
( without meaning)
-这儿是哪儿啊？
-你们哥儿两过会儿去玩儿吗？
-Will you two guys go to play after a while?
Do have meaning:
-你（的）儿几岁了？
-How old is your child/children?
-你真是我的好大儿啊！
-You are really my good child!
——————————
I hope my answer can help you :). If you still have doubt about this topic, I will listen.
Uhmm~That's a good question.I am sure that all of them do have a meaning of 'Where...' as you said. Generally speaking, they can swap with each other in daily life (and this always happen), which make it easy for me to explain all these three at the same time. Yeah there are no big differences BUT small ones.In my mind, those sentences are not clear if you just give me them without any situations. “这” is a pronoun. It can refer to many many things, which makes these sentences have wild meanings. Their relationship:“这是在哪儿”＝“这是在哪”＞“这是哪”————————————Examples here~1. I come to a place that I don't know so I am really confused, or I only know a name of someplace. I may ask “这是（在）哪（儿）”. “这” can only refer to the PLACE. And I need more details of this place. The details includes the location and the environment...Just anything related to the place. We often use “这是哪” in such a situation.-（开车开到了荒地。）-( Drive to nowhere.)-这是（在）哪（儿）？-(Where am I ???/ What is this place ???)-汤姆去M国了。-Tom went to M country.-这是（在）哪（儿）？-Where/What is 'M country'?2. When “在” appears, the question is not only able to ask for the 'details of some places', but also the location of obejects/people/cases... “这” can refer to anthing we like.-珍妮打架了。-Jenny got into a fight.-这是在哪（儿）？-Where did the fight take place?-看这张海滩的照片。-Look at this picture of a beach.-这是在哪（儿）？-Where is the beach?-我找不到我的帽子了, 这是在哪（儿）？-I can't find my hat. Where is it?-我找了，给。-I find it! Here you are.-谢谢。这是在哪（儿）？-Thanks. Where was it?/Where did you find it?3.“这是在哪”and“这是在哪儿” don't have differences in meaning. “儿” doesn't have meaning in the case that it is a kind of accent or talking habit.If you want, you can add “儿” after every word XDD. That's not a big deal. You may choose where to add or not to. Remember to say it softly with no tone, or other people may misunderstand “儿” as “儿子(son)”、“女儿(daughter)”、“孩儿(child/children)”.Habit & Accent: ( without meaning)-这儿是哪儿啊？-你们哥儿两过会儿去玩儿吗？-Will you two guys go to play after a while?Do have meaning:-你（的）儿几岁了？-How old is your child/children?-你真是我的好大儿啊！-You are really my good child!——————————I hope my answer can help you :). If you still have doubt about this topic, I will listen.
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