How to make 3 random numbers that sum to 1

I'm trying to make two matrices for 6 random numbers, and the three different numbers of each need to sum up to 1. ex)P1+P2+P3=1 . How can I make this?? thanks. this is some part of my script file.

P1=rand(1,1)
P2=rand(1,1)
P3=rand(1,1)
Q1=rand(1,1)
Q2=rand(1,1)
Q3=rand(1,1)
    if (P1+P2+P3==1) && (Q1+Q2+Q3==1)

          %setting P, Q matrix
              P=[P1 P2 P3];
              Q=[Q1; Q2; Q3];

Answer 1

[Image Analyst]

Did you try this:

r = rand(1, 3); % Start with 3 random numbers that don't sum to 1.
r = r / sum(r)  % Normalize so the sum is 1.
theSum = sum(r)  % Check to make sure.  Should be 1

[Shuva Paul]

Helpful! Thanks :) :)

[krishna vasishta]

It works!! Many thanks!!!

[CLOUDY LU]

Wonderful idea! Thanks!

Answer 2

[Roger Stafford]

If you want your results statistically uniform area-wise, you can use the 'randfixedsum' function in the File Exchange at:

https://www.mathworks.com/matlabcentral/fileexchange/9700-random-vectors-with-fixed-sum