Adds two magnetic fields I have two magnets attached to the table so that they can't stick together. Then, I put the iron in the middle of the two magnets, is the  equation of magnetic field that the iron feels is only B1-B2?
 A: If your question is, Does iron experiences only superposition of magnetic fields from those two permanent magnets,- then answer is NO. Because iron is ferromagnetic material - it becomes a magnet too, so it experiences his own magnetic field too. The answer is that basically three magnetic fields participates in superposition. Check magnetic field lines :

A: You can think about this problem from the analogy of electric dipole.
As more or less they go parallel.
Here Iron itself will become a magnet.so you got three dipoles.
You have three electric ( assumed) dipoles placed side by side. Now imagine you have dipole 1 , dipole 2 , dipole 3 .
Its quite simple , three dipole are placed at axial position with respect to each other . So the direction of force will be along axial direction only . So the net force will be difference of force due to two magnetic field.
Remember we don't  to consider the force on magnet due to its own field in such type of questions.
And avoid extreme rigour cases such that one pole will be more stronger due to strong magnetic field.
A: Magnetic field $\vec B$ is a vector quantity. This means that if at any point the net magnetic field will be a vector addition of magnetic field at that point due to all sources. 
$$\vec{B}_{net}=\sum_{i}^{\text{source}}\vec B_i$$
Thus way the magnetic field at any point $\vec r$ in your example will be given by 
$$\vec{B}_{net}\left(\vec r\right)=\vec B_1\left(\vec r\right)+ \vec B_2\left(\vec r\right)$$
However, like @Agnius pointed out, if you keep a ferromagnetic material in your region of interest, there will be a new magnetic field in the region coming due to the induced field $\vec B_{ind}$ in the ferromagnet. Here the net field will then be 
$$\vec{B}_{net}\left(\vec r\right)=\vec B_1\left(\vec r\right)+ \vec B_2\left(\vec r\right)+ \vec B_{ind}\left(\vec r\right) $$
