Our advice would be to remove the cherry trees if you have broodmares. They pose the most serious threat. If you want to remove the black walnut trees, they could be sold for furniture wood. The black locust trees make an excellent source of firewood, if you decide to remove them. The rest of the answer provides more information to help you decide how many of the trees to remove.
In most cases, if your horses have access to adequate pasture or hay, they will avoid consuming plants that could be toxic to them. Many horses live right alongside all the trees you mention without any problems.
Horses may be affected by black walnut chips or sawdust when these are used for bedding material. However, it is advisable to remove downed limbs from the pasture to prevent the horses from eating the dried leaves. Close association with walnut trees while pollen is being shed (typically in May) also produces allergic symptoms in both horses and humans. The juglone toxin occurs in the leaves, bark, and wood of walnut, but these contain lower concentrations than in the roots. Juglone is poorly soluble in water and does not move very far in the soil. Black walnut trees are very valuable depending on the size, so before you remove them from the pasture, you might want to find out about selling the trees for furniture wood, etc.
Wild cherry trees pose a different problem. They attract eastern tent caterpillars that were determined to be the cause of Mare Reproductive Loss Syndrome (MRLS). You can read more about MRLS here. Many horse farms in central Kentucky removed their wild cherry trees to decrease their mares' exposure to the caterpillars and reduce their risk of late-term abortion.
Wilted wild cherry leaves can lead to cyanide poisoning. Poisonings occur most frequently when wilted leaves are eaten but have also been reported when leaves are consumed directly from the tree or sprouts or in dried hay. The amount of hydrocyanic acid formed once the plant material is ingested is affected by the type of stomach juices and the kind of feed the animal had previously consumed. Ruminant animals appear to be more susceptible to poisoning than horses. Removal of downed limbs from the pasture reduces the risk of the horses consuming the wilted leaves.
Several toxic compounds are found in black locust sprouts, leaves, bark, flowers, and seed pods. Horses are susceptible to the effects of black locust. Poisoning and death have been reported for horses consuming bark, leaves, or sprouts. Affected horses often stand with feet spread apart. Other signs include depression (often extreme), diarrhea, anorexia, weakness, posterior paralysis, colic, pupil dilation, coldness of extremities, laminitis, weak pulse, and rapid, irregular heartbeat. Normally, the horse won't die from the poisoning, but, in severe cases, death can result from cardiac failure.
