פַּ֣ר אֶחָ֞ד בֶּן־בָּקָ֗ר אַ֧יִל אֶחָ֛ד כֶּֽבֶשׂ־אֶחָ֥ד בֶּן־שְׁנָת֖וֹ לְעֹלָֽה׃
一頭公牛犢，一隻公綿羊，一隻一歲的公羊羔，作為燔祭；
Rashi
פר אחד ONE BULLOCK — i.e. that which is unique (מיוחד) in the herd (cf. Sifrei Bamidbar 50).
Chizkuni
פר אחד בן בקר, “one young bullock;” compare the word בן as describing something when used in connection with pigeons, i.e. בן יונה.
Siftei Chakhamim
The best. Here the Torah does not mention any number that is greater than one such that it had to explicitly write the word “one” to distinguish this from the other number. Rather this came in order to expound. Another answer is that because the Torah deviated, writing “one bull, young” and did not write “a young bull, one” (as with the other animals in this verse) this certainly came in order to expound.