Pattern help.

[Shahr]

What does this mean?

Can you please explain what they mean by slip stitch join into 3rd 2ch. I do not understand the 3rd 2ch part. 
Thanks!!

[BeautifulNightmare]

I’m assuming it means you are joining to a ch up

so at the beginning of a round say it says “ch 2, dc in each st around” at the end of the round you use a slip stitch to close the round you just made.  So you slip stitch into the ch 2 you made st the beginning of the round.

[Granny Square]

I agree with the above poster in that it's telling you to join the end of the round in the beginning by slip stitching into the top of the first stitch (or chain up that stands in for a stitch), but think the '2' might be a typo OR you are changing stitches in the next round.

If you are working in (US) DC, 99% of the time the turning chain is given as 3 chains, and for DC the 3 chains count as a stitch; when you join a round of DC, you slip stitch into the top (third) chain.  If the next round is DC, the 2 makes no sense.  It should have said 'slip stitch join into 3rd ch' and then probably "chain 3" (to start the next round).

If the turning chain for the current  round was only 2 (which can happen with HDC), then the "3rd" makes no sense; HDC is usually chain 1 or 2, and sometimes it does and doesn't count as a stitch.  What would make sense in the instruction you gave is not a typo, is if you were finishing a round of DC, and starting a round of HDC.  However, if so it's written in a rather abbreviated way; it would have been more clearly written as "slip stitch join into 3rd ch at start of round, ch 2", HDC....