Exodus 34:23

Thrice in the year shall all your men children appear
before the Lord God
At the three above mentioned feasts, (See Gill on Exodus 23:17) here it is added,

the God of Israel;
who had chosen them to be his special people, had redeemed them out of Egypt, and done great things for them since; had made a covenant with them, and had now renewed that covenant with them, and was their covenant God, and they his people, and so were under great obligations to present themselves unto him at the times appointed by him.