Use matlabFunction with the 'Vars' option that gives a vector of variable names within a cell array, rather than a vector of variable names. Any names specified in a vector inside a cell array are bundled into a vector rather than as individual inputs.
For example,
f = matlabFunction(x^2+y,'Vars', {t, [x,y]})
To be more complete: the items are put into an array and the appropriate column of the array is pulled at, so the generated function can be used as vectorized by putting different input value combinations in different rows. The ode* routines pass inputs as a column vector, but the generated code will expect the inputs (for any one case) as a row vector. You will need to transpose, so...
f = matlabFunction(Y,'Vars', {t, [theta1p; theta2p; theta3p]} );
[t,y45] = ode45( @(t, y) f(t, y(:)), tspan, initcond)
