John the Beloved Apostle was actually there and he says that there were only two–one on each side–see John 19:18:

-Here it is transliterated in Greek:

18 hópou autón estaúroosan kaí met autoú állous dúo enteúthen kaí enteúthen méson dé tón Ieesoún.

-Here’s my rough literal translation:

Where him they crucified, and with him others two, on this side and on that side(one), and in the middle Jesus.

-Here’s the ESV -probably the most accurate modern English translation:

18 There they crucified him, and with him two others, one on either side, and Jesus between them.

It appears that these “discrepencies” can be easily resolved, as with almost all apparent difficulties in Scripture, with a careful reading of the Greek Originals. There are not just two, but four, that were crucified with our Lord. In Matthew and Mark’s account, the persons are identified as “lestai”, the Greek word for robbers, and in Mark’s testimony the word is “kakourgoi” = malefactors.

There is no basis for the assumption that there was only two others involved. That is purely a traditional stance, not found in the Word of God.

The first two were “led with Him to be put to death” (Luke 23:32). When they were come to Calvary, “they crucified Him, and the malefactors, one on the right hand, the other on the left” (v.33)

After He was crucified, along with the two malefactors, and AFTER they parted His garments, and “sitting down they watched Him there”, “then were there two robbers crucified with Him, one on the right hand, and the other on the left” (Matt 27:38, Mark 15:27)

Additionally, when they broke the legs of the first and then the other, they did not pass by Jesus, but “having come to Jesus, and saw that He was dead already, they brake not His legs” (John 19:32,33).

It is of the utmost importance to view the inspired Autographs as close to the Originals as possible in order to be accurate and avoid error, such as in this instance of assuming that there is only mention of two others when according to the Holy Writings there is four. A thorough examination with a Greek cocordance will yield unending joy to the serious student of God’s revelation to us.