Scratch

Hmm... I see what you mean. Perhaps try something like this.

when gf clicked
ask [first number] and wait
set [#1 v] to (answer)
ask [second number] and wait
set [#2 v] to (answer)
ask [third number] and wait
set [#3 v] to (answer)
set [total v] to ( (#1) + ( (#2) + (#3) ) )
say (join [The total is ] (total) )

The way with lists (unlimited amount of inputs)

when gf clicked
ask [how many numbers?] and wait
set [numbers v] to <answer>
set [1 v] to [0]
repeat (numbers)
change [1 v] by [1]
ask <join<join [what is number] <1>>[ ?]> and wait
add <answer> to [list of numbers v]
end
repeat (length of numbers)
replace item (1 v) of [list of numbers v] with <<item (1 v) of [list of numbers v]>+<item (2 v) of [list of numbers v]>>
end
say <join <item (1 v) of [list of numbers v]> [ is the total]>