Use one extra variable and do your processing outside the loop in the third case.

E.g.

declare @TestVar = 0

>I need to do something like the following in a query.
>
>

>
>DO WHILE looping through fetched rows and @MyVar changes value
>
>BEGIN
>
>IF @MyVar = 1
>     BEGIN
>          Do This
>     END
>
>IF @MyVar > 1
>     BEGIN
            set @TestVar = 1
  
>          Do This
>     END
>    >

IF @TestVar = 0

&& All your myVar are 1 (or less than 1), so you can try extra processing.