Matlab - randomly generated seeds only spawning one of the 3 desired features - matlab

Question

I am trying to model distributions of randomly sized craters in a given landscape. I have 3 diffrent crater sizes. Big ones that spawn infrequently and only during the first half of the model. Medium ones that spawn at a 'normal rate' only during the second half of the model. And small ones that spawn frequently during the whole model.
My issue is that only small ones are spawning. I think it's linked to how I use 'rand' to decide what should spawn. I basically have the following structure:
%medium craters
icrater=0;
for k=nstep/2:nstep
Time_in_Myr=k*dt/1e6;
med_c=rand
if med_c>0.98
icrater=icrater+1;
x_crater(icrater)=dx*(rand*nx);
y_crater(icrater)=dy*(rand*ny);
t_crater(icrater)=k;
end
end
%big craters
bigcrater=0;
for k=1:nstep/2
Time_in_Myr=k*dt/1e6;
big_c=rand
if big_c>0.991
bigcrater=bigcrater+1;
x_crater(bigcrater)=dx*(rand*nx);
y_crater(bigcrater)=dy*(rand*ny);
t_crater(bigcrater)=k;
end
end
%small craters
smallcrater=0;
for k=1:nstep
Time_in_Myr=k*dt/1e6;
small_c=rand
if small_c>0.97
smallcrater=smallcrater+1;
x_crater(smallcrater)=dx*(rand*nx);
y_crater(smallcrater)=dy*(rand*ny);
t_crater(smallcrater)=k;
end
end
So what I think it's doing is only spawning small craters because they have the highest spawn rate.
I tried adding the 'med_c=rand' (and equivalent) bits to allow for the code to check the if statement wiht the same random number.
I was expecting to check if each 'if' statement was true, and if one or more of them were, to spawn those craters. So I would end up with a handful of large ones, ten or so medium ones, and a lot of small ones. But I only got small ones.