John Trapp on Psalms 20:3

Verſ. 3. Remember all thine offerines] Before they went to war, they were wont to offer ſacrifices, as did Saul; though by his over-haſte therein hee loſt his Kingdome, 1 Sam. 13.9. From the people of God, the Heathens alſo had learned this courſe and cuſtome. Io Paean is in force Jehovah Peneh, that is, Lord look upon us. The Devil (Gods ape) had taught the Athenians, when they began the battel, to uſe theſe words, and thereby to hearten one another. The Romans alſo did the like; and would not deſiſt till they had an auſpicious anſwer; hence alſo they called a ſacrifice hoſtiam, becauſe, when they went againſt their enemies, they offered it.

And accept thy burnt ſacrifice] Or, turn it to aſhes (and ſo ſeal up his acceptance) or, make it fat, i. e. take delight in it, as men do in fat things full of marrow, ſaith R. Solomon.

Selah] This is added, to ſhew, ſaith Vatablus, with how great fervency the people ought to pray for their King.