Numbers were often imbued with a mystical significance. For example, 40 with spans of time indicated that the time was chosen by God. 40 days and nights of the flood, the Israelites wandering in the desert for 40 years, Jesus going into the desert for 40 days.

People in groups of 12 were chosen by God. 12 tribes of Israel, 144,000 (12x12x1000) saved in the Rapture, 12 apostles, etc.

3 and 7 were similarly associated with God. Tions of examples.

Just picking a small nit: Matthew is not considered the oldest gospel, Mark is. There may be fragments of Matthew that are the oldest, but it is generally agreed that the writer of Matthew was influenced by the already-written gospel of Mark. Luke was written around the same time as Matthew, or a few (5-10) years later.