Two white dwarfs with a total mass >1.4$M\odot$ merge. It may trigger a type Ia SN explosion.

How about the three cases below?

1 two low mass WDs merge, total mass < 1.4$M\odot$

2 a low mass WD + a low mass neutron star, total mass < 2$M\odot$

3 a low mass WD + a neutron star, total mass >2$M\odot$

If any of the above can form a SN, we have observed this kind of SN?