I noticed that
the only time 3s appear is all by themselves and at the end of the message. That implied to me that they were spaces (and the last part was an "end of message" mark).
The next step was
to replace them all with letters: this gives
A CDEF G CAHHCF IJFFHKFLMH. Plugging this into quipqiup, an online cryptogram solver, gives a top result of
I LOVE M LITTLE SWEETHEART. The
M is likely meant to be a
U, judging by the context of the message (since we don't have anything else to distinguish). So the message was I LOVE U, LITTLE SWEETHEART. (How cute!)
HH may have been the initials of either the sender or receiver of the message.
So how were we supposed to figure it out? I'm not entirely sure. Here's a list of symbols and letters from shortest to longest.
More common letters have shorter codes, which is fairly reasonable. But it doesn't seem to have any obvious structure other than that, as far as I can tell. It may have been something like Morse.