How do I use tr command to replace: full stops, exclamations and question marks followed by a space, with newlines?

You don't. tr is not designed for that. It's designed to transliterate a set of single characters into another set of single characters, e.g., A-Z into a-z. Using tr with .␣ (a dot and a space) and \n will replace all dots and spaces with newlines.

Use (GNU) sed instead:

$ echo 'I am happy. I am here. How are you, Meg?' | sed 's/\([!.?]\) /\1\n/g'
I am happy.      
I am here.       
How are you, Meg?

The sed editing script here will replace all occurrences of !, . or ? that are followed by a space, by the same character and a newline.

As far as I know tr only works with single characters and ". " is a string not a character, so it is possible to do what you want by using sed or awk, for example:

sed -e "s/\. /\n/g" file.txt > out.txt