The three numbers are minimum size, priority, and maximum size.
If there is enough disk space to create this partition and all the partitions with a higher priority value with their maximum sizes, and still enough to create all defined lower-priority partitions with at least their minimum sizes, then you'll get the maximum size.
Basically, partman-auto starts with taking the minimum sizes of all the partitions. If the disk is not big enough to satisfy them, the partitioning fails and installation stops there.
If the disk is at least big enough to implement the minimum sizes, then any remaining disk capacity is divided up between all the partitions, with a weighted distribution. The weight for a given partition is its priority number minus its minimum size. Any partitions that would be sized larger than their maximum size due to this divvying up of the extra capacity, will instead by set to their maximum size, and then the leftover capacity will again be divided in this way until either all of it is used up or all partitions are set to max capacity. (See description in the docs for the exact method used.)
Once the sizes are determined this way, partman-auto has found the right partition sizes and actually does the partitioning.
Ubuntu uses the same installer technology as Debian does, so here's as official documentation as it gets: https://github.com/xobs/debian-installer/blob/master/doc/devel/partman-auto-recipe.txt
