The first one, on the left, is a part of a set of seven stamps about mushrooms and issued on the 1st of December 1985. The stamp pictures the Amanita caesarea, commonly known as the Caesar’s mushroom, belonging to the genus Amanita, that can be found in Southern Europe and North Africa. Its common name comes from the fact it is supposed to be the favourite of Roman emperors ! The mushroom has an orange/red cap and a smooth surface. It is an edible mushroom but that can be easily confused with some poisonous members of the Amanita genus such as the Amanita muscaria.Here is a picture of the seven stamps of the set. I only found a picture of a used set.
When I see these stamps I think that they look more like stamps issued to please topical collectors than stamps issued for a real postal usage. So this is interesting to see them on a cover.
The second stamp, the one in the corner, belongs to a set of seven air-mail stamps commemorating Mongolian National achievements which was issued on the 11th of July 1977. The stamp pictures a red cross ambulance and an hospital. I’m not sure to understand which achievement is considered here… I could not find any picture of the full set…
The yellow symbol on the left side of the stamp is a symbolic representation of the coat of arms of Mongolia. I do not know what is the black symbol just below.
The last stamp is a part of a set of seven stamps picturing African animals (it seems Mongol Post likes to issue stamps per set of seven !) and issued on the 23rd of May 1991. The stamp on the cover pictures a Gorilla. Here is a picture of the full set.