if-elseif-else statement take turns/used each of them in while loop, how to write properly?

Hello, im new in MATLAB, and here's my problem: the while loop should be run at sometime, while each loop should take the if statement sequentially (while loop do work> take if statement>second loop>take elseif > 3rd loop> take else> 4th loop> if statement> and so on) i kinda new to use some of the feature like rem feature. Could you give me some guidance? thanks :)

i=1
a(i)=0
while true
    i = i+1
    a(i) = a(i-1) +1
    if a==1:3:15 
        disp('its first num')
    elseif a== 2:3:15
        disp('its sec num')
    else a==3:3:15
        disp('its 3rd num')s
    end
    if a > 20
        break
    end
end

Answer 1

[Voss]

Like this?

i=0;
while i <= 20
    i = i+1;
    r = rem(i,3);
    if r == 1
        disp('its first num')
    elseif r == 2
        disp('its sec num')
    else
        disp('its 3rd num')
    end
end

its first num

its sec num

its 3rd num

its first num

its sec num

its 3rd num

its first num

its sec num

its 3rd num

its first num

its sec num

its 3rd num

its first num

its sec num

its 3rd num

its first num

its sec num

its 3rd num

its first num

its sec num

its 3rd num