1) They had no plan to press against the back 3. Arsenal always had an easy square pass to an unmarked defender to get away from the highest pressing forward.
2) Playing a narrow diamond in the first half against wingbacks was a huge mistake. Arsenal had numbers on the wings and an easy path to progress the ball forward because there were no natural markers for the wingbacks. In theory the flipside should have been that Spurs had control of the center with four CMs but with Sissoko and Dier as the two deepest midfielders they didn't have enough quality passing in tight spaces to take advantage of it.
3) Poch tried to compensate at halftime by switching to a back 3, with Dier dropping into defense and Eriksen dropping deeper. This allowed Spurs' fullbacks to more directly match up with Arsenal's wing backs. But it also conceded control of midfield (partly because Dele played curiously high for unclear reasons) to an Arsenal midfield 3 willing to take up aggressive positions, as Spurs now had Sissoko and Eriksen in a two against Ramsey/Torreira/Xhaka.
Now it didn't help Poch that there were a lot of poor performances today. Dele was almost completely invisible offensively and did nothing off the ball to slow down Arsenal's midfield. Dier, Vertonghen, and Foyth all made stupid individual errors that led directly to goals. Aurier and Davies were not good. Kane did very little. Arsenal scored two from outside the box, one deflected, which is sort of hard to guard against. But even apart from that, except for a 15-minute spell before half time, Spurs struggled to get a grip on the game and were always on the back foot.