When you click the mouse, the condition 'if mouse has been clicked' remains true for the whole event sheet until the next tick, not just for the first event that uses it. The simplest ways to fix it are to either order those events backwards or set a variable mouse clicked to 1 and check the variable in each event, like:
1: On clicked Button:
variable 'mouseclicked' = 0
a.Write Text
b. Button.Set"ChoiceA" to 0
c. set variable 'mouseclicked' to 1
2. On clicked Button:
variable 'mouseclicked' = 0
Value "ChoiceA" equal to 0
a.Write second Text
b.Button.Set"ChoiceA" to 1
c. set variable 'mouseclicked' to 1
3. On clicked Button:
variable 'mouseclicked' = 0
Value"ChoiceA" equal to 1
a.Write third Text
b. set variable 'mouseclicked' to 1
Then set the mouse clicked variable to 0 at the end of the event sheet.