There are at least 4 problems in your code:
1. Split your main code and your ode function:
tspan = [0 10];
f0 = [0 0];
f10= [10 1];
[t,f] = ode45(@myfcn, tspan, f0, f10);
function df = myfcn(t,f)
df = zeros(3,1);
df(1) = f(2);
df(2) = f(3) ;
df(3) = (-.5*f).*f(3) ;
2. There is an initial condition needed for every ode. Since you have 3 equations you have to give f0 as 1x3 vector, not as 1x2.
3. If you want to use f10 as boundary condition ode45 is the wrong tool. To solve boundary value problems use bvp4c. For the usage of ode45 leave it away.
4. The last equation has f*... - f is a 3x1 vector, which will cause problems, because df(3) is expected to be a scalar. Maybe you mean f(1) for example.
Making these corrections should give you an error free code. If it is meaningful in the end i can not say.