The three packages just happen to be identical at this moment in time.
python27 installs the latest stable version of Python 2.7.x
python2 installs the latest stable version of Python 2.x.x, which will be Python 2.7.x until Python 2.8 is considered stable enough to replace it.‡
python installs the latest stable version of Python, which happens to be the same as the above, presumably until Python 3 is considered stable enough to replace it.
So you need to decide which version numbers are important to you, and which you are happy to track.
But as I said at the start, looking at the FreeBSD website, all three currently install Python 2.7.12.
Generally, anything before the hyphen is part of the FreeBSD package name, and anything between the
, is the actual version of the item. Any number after a ',' represents a bump in the (FreeBSD) package version where the item's version has not changed.
‡ Although as Roland Smith commented, this is unlikely to happen.