Will anything (gameplay wise) be missed in 3 if one does not play the DLC in 2?

Yes there is.

If you don't have the DLC for Kasumi you will miss out on her interaction for the hanar mission:

Kasumi reappears on the Citadel, giving an anonymous tip to the salarian Spectre Jondum Bau about hanar operatives being aware of an Alliance black ops raid into batarian space. With Shepard's assistance, they were able to corner the hanar, who was willingly serving the Reapers. Kasumi was able to stop the hanar from uploading a virus that nearly disabled Kahje's defenses, at the same time faking her death. She was then convinced by Shepard to lend her assistance with the Crucible .

Zaeed, will also make an appearance in the game on a quest involving a Hanar and Cerberus, resulting in the volus bombing squadron as a war asset.

Other smaller dialog changes include conversations with Liara (shadow broker related), and the outcome of the Arrival DLC.

If you saved Gavin Archer's brother, David, in the Overlord DLC and let him stay at Project Overlord, you'll run into Gavin again in ME3 on a side quest involving Jacob and he will be a war asset.

Other DLCs such as Bring down the Sky from ME1 will allow you to increase your war assets. In this case, if you let the Batarian leader live, you can recruit his fleet.