It is somewhat similar to what I've seen in English where we refer to random people as "Tom, Dick, and Harry" or Spanish where it is "Fulano, Mengano y Sultano".
First, regarding this, I must say that Japanese doesn't have a parallel idiom using real given names. If I had to list, there are 太郎 (male) and 花子 (female) which are often used as placeholders, so maybe you can say like 太郎も花子も "all Taro and Hanako", but it'd be by no means idiomatic nor customary. If you look for the counterpart for "every Tom, Dick and Harry", consider 誰も彼【かれ／か】も "one and all", 誰彼問わず "no matter who", or a little funny-sounding idiom 猫も杓子も lit. "all cats and ladles" which somewhat means "everybody and his dogs".
I would have expected them to be いちろう, にろう, さんろう, and しろう. If いち is 1 and し is 4, what are じ and さぶ? Is this some way of counting that I don't know about?
Then back on topic, the readings of 二郎【じろう】 and 三郎【さぶろう】 are indeed irregular, because they have history. Traditionally, these numbering names (輩行名) did not start from 一郎【いちろう】 but 太郎【たろう】→次郎【じろう】→三郎→四郎… Here 太 roughly means "primary" and 次 "secondary", not numbers per se.
二郎's reading is simply borrowed from 次郎. 二 certainly has a pronunciation じ as 漢音, so not totally inexplicable, but that one is rarely used in counting. Incidentally, 二乗 "2nd power, square" is also often read じじょう, which actually comes from that of 自乗 "multiplies itself".
三郎 is a result of phonetic shift. 三 was originally read "sam" instead of "san", as it was in Middle Chinese. So its early phonetic form was something like さむらう. However, in Japanese //m// and //b// sometimes switch with one another (similarly to how Ham-shire has become Hampshire): さみしい vs さびしい, けむる vs けぶる, かまぼこ vs かばやき etc. Due to the alteration, this word has been eventually fixed on the pronunciation さぶろう.