First, you should combine your three "prom" ode functions into one
IC = [n0 m0 h0];
[t, NMH] = ode45(@(t,nmh) prom(t,nmh,V1), tspan, IC);
n = NMH(:,1):
m = NMH(:,2);
h = NMH(:,3);
...
where your ode function is something like
function dNMHdt = prom(t,NMH,V1)
n = NMH(1);
m = NMH(2);
h = NMH(3);
dndt = ...
dmdt = ...
dhdt = ...
dNMHdt [ dndt; dmdt; dhdt];
end
Then you can think of doing something along the lines of:
V1 = [20 40 60 80];
for i = 1:4
[t, NMH] = ode45(@(t,nmh) prom(t,nmh,V1(i)), tspan, IC);
n(:,i) = NMH(:,1);
m(:,i) = ...
end