What is the meaning of "Shalosh Regalim"?
Rashi, Targum Onkelos and Targum Yonasan: It means three times a year.
Chagigah : It means three foot-festivals, on which one is obligated to go up to the Beis-ha'Mikdash on foot.
Chagigah, 4a: Refer to 23:14:1:2. Consequently, someone who has a wooden leg,
who is lame (even in one leg
See Torah Temimah, note 94. The Gemara there also learns it from the words "Shalosh Pe'amim" in Pasuk 16, which it equates with "Shalosh Regalim".
Why does the Torah insert the Parshah Shalosh Regalim in the Parshah of Sh'mitah?
Mechilta: To teach us that, even in the Sh'mitah - when the various Melachos connected with the produce do not apply,
one is still obligated to celebrate the Yamim-Tovim.
See Torah Temimah, note 96.
Oznayim la'Torah: This teaches us that, rejoicing on Yom-Tov, as well as learning Torah on Shabbos (Refer to 23:13:4:1) guards Yisrael from Avodah-Zarah (as hinted in Ki Sisa, 34:17 & 19
What are the connotations of the words "Tachog Li"?
Seforno: 'Celebrate (rejoice) with Me (Hashem).
Targum Onkelos and Targum Yonasan: 'Celebrate before Me (Hashem)'.
As the Pasuk writes in Tehilim, 149:2, as opposed to the Pasuk in Sh'mos 32:19 "Vayar es ha'Eigel u'Mecholos" (Seforno).