Rashis comment on Shemot 23:19 is:
You shall not cook a kid: Heb. גְּדִי. A calf and a lamb are also included in [the term] גְּדִי, for גְּדִי is only an expression of a tender young animal. [This you know] from what you find in many places in the Torah where גְּדִי is written, and it was necessary to write after it עִזִים [to qualify it as a kid], for example, “I will send you a kid גְּדִי עִזִים ” (Gen. 38:17); “the kid גְּדִי הָעִזִים ” (Gen. 38:20); “two kids עִזִים גְּדָיֵי ” (Gen. 27:9); to teach you that wherever גְּדִי is mentioned unqualified, it also means a calf and a lamb. This [prohibition] is written in three places in the Torah, one for the prohibition of eating [meat with milk], one for the prohibition of deriving any benefit [from meat with milk], and one for the prohibition of cooking [meat with milk]. -[From Chul. 113b, 115b]
http://www.chabad.org/parshah/torahreading.asp?AID=15564&p=5&showrashi=true
Rashis comment on Shemot 34:26
You shall not cook a kid: This is the warning against [cooking] meat and milk [together]. This commandment is written in the Torah three times (Exod. 23:19, Deut. 14:21), one for eating, one for deriving benefit, and one for the prohibition of cooking. -[from Chul. 115b]
a kid: Heb. גְּדִי. Any young offspring is meant, even a calf or a lamb. Since [the Torah] had to specify in many places גְּדִי עִזִּים [when a young goat is meant], you learn that [mention of] גְּדִי unqualified means all sucklings. -[from Chul. 113b]
in its mother’s milk: This excludes fowl, which has no milk, which is not prohibited by the Torah but by the decree of the Scribes [the Sages]. -[from Chul. 113a]
http://www.chabad.org/parshah/torahreading.asp?AID=15567&p=6&showrashi=true
Rashis comment on Devarim 14:21
You shall not cook a kid [in its mother’s milk]: [This is stated] three times [here, in Exod. 23:19, and in Exod. 34:26], to exclude wild animals, fowl, and unclean animals [from the prohibition of cooking meat in milk]. — [Chul. 113a]
http://www.chabad.org/parshah/torahreading.asp?AID=36235&p=4&showrashi=true