A 2-bladed wind turbine is less stable mechanically than 3 (or more) blades.
Because the two blades are in line, it is much easier to twist the hub of the turbine in the direction along the line of the blades than to twist it at right angles to the blades. If the turbine is being rotated because the wind direction changes, this will cause an unbalanced twisting force on the hub (and the pole) at twice the rotation frequency of the blades.
This unwanted twisting of may feed back into the blades causing them to vibrate. If the speed of the twisting matches the natural vibration frequency of the blades, this could lead to catastrophic mechanical failure. As a consequence of this, 2-blade turbines usually have to be rotated slower than those with more blades, if the wind direction changes, but the wind blowing in an off-axis direction also produces the same unwanted twisting effects. Catch-22!
The hub mechanism and/or the pole itself may need to be designed stronger or stiffer to resist these unwanted twisting forces.
With 3 or more blades, the mechanical behaviour of the turbine is actually the same in all possible orientations of the blades. Even if there are even number of blades, for example 4, there is nothing "special" mechanically about the 90-degree angle between two adjacent blades but economically 4 blades are more expensive to make than three.
The mechanical issues with 2-blade turbines can be "designed out," but the optimum number of blades to generate the maximum power for a given size of turbine is typically 4, ignoring considerations like the extra weight of more blades. As such, a 3-blade design is often the best "minimum cost" compromise between a simple and reliable design, high power output, and low weight.