Three Times a Year

Why are we to appear before God three times a year, “at the Feast of Unleavened Bread, the Feast of Weeks, and the Feast of Tabernacles”? What is meant by this term, since we are commanded to attend every Holy Day service, in addition to weekly Sabbath services? How are we to appear, and where? Why does God command to give an offering on each Holy Day? Does He need our money?