The Giants enter Phase 3 of their offseason program Monday with the first of 10 organized team activity practices, a time when the football work on the field gets ratcheted up and the coaching staff learns more about what it has, and does not have, on the roster.

Here are five of the most topical areas for the Giants as they move forward:

This team is set with Odell Beckham Jr. and Sterling Shepard as the top two receivers, but the identity of the No. 3 receiver — essentially another starter, given how much the NFL nowadays is in a three-receiver set — is a mystery. Roger Lewis (36 catches in 2017) has a shot, as does Cody Latimer, a big guy without much production (35 career receptions) in four years with the Broncos. Travis Rudolph returns after spending most of last season on the practice squad.

“It’s a little bit easier to catch the ball versus air, so we’ll start to be able to evaluate the receivers as a whole once we can be as competitive as we can be with the OTAs,’’ head coach Pat Shurmur said. “So I think that will start to separate what we think of the wideouts. There are guys on our roster that we’re going to develop a trust in and put them out there and we expect them to produce.’’

Shurmur maintains the Giants are “a better team with him here than without him,’’ and now Ereck Flowers gets to prove if that statement is valid. The erstwhile starting left tackle last week finally decided to grace the team with his presence, and he gets thrust into the competition for the starting right tackle spot, where second-year Chad Wheeler has worked this spring. This will not be an easy transition for Flowers, a left tackle in college at Miami and in his first three years in the NFL. Expect him to struggle to get his bearings. These practices should show what he needs to work on leading into more grueling training camp tests.

There is no truth to the rumor the Mets are in discussions with Beckham to have him replace Juan Lagares in center field (you know Odell could cover a whole lot of ground) and as a hitter in the lineup (check out the footage of Odell swatting home runs at Tampa Bay Rays batting practice — love the ripped-at-the-knee jeans and the throwback Devil Rays jersey). Beckham will attend the OTAs but has not yet been cleared for full participation coming off a fractured left ankle. Any sight of him on the field, though, is worth nothing because any sight of Odell doing anything anywhere is worth noting.

The battle for roster spots at cornerback behind Janoris Jenkins and Eli Apple figures to be fierce, with veterans picked up from around the league and youngsters added after the draft vying for spots. It sure looks as if a rookie waiting in the shadows has a shot here to make the team. Two possibilities are Aaron Davis and Grant Haley, both undrafted, arriving after notable careers at big-time college programs. Davis has good size (5-foot-11, 194) and started 45 games at Georgia, including all 15 last season for the national runner-ups. Haley started 36 games at Penn State, was a team captain and at 5-9 and 190 pounds could make some noise as a slot corner.

“Certainly we’re going to depend on those younger players to really get in the mix and try to make the team,’’ Shurmur said of this crop of rookie cornerbacks.

These practices, with no live hitting and no threat of getting crushed by a pass rusher, are usually quarterback-friendly affairs. Just how friendly these sessions become for Davis Webb and Kyle Lauletta should make for interesting viewing. This new coaching staff is learning about Webb — there’s only so much to be gleaned from watching 2017 practice tape — and this is a time for Webb to display enough skill and smarts to strengthen his hold on the backup role behind Eli Manning. He knows he was drafted by the previous regime, with the current regime selecting Lauletta in the fourth round last month.

“You can tell by the way he handles his business that he’s going to be able to pick everything up very quickly,’’ Shurmur said. “A lot of times in coaching, it’s kind of like a quick drive-by, you want to make a point and keep moving, he’s able to take it in and run with it.’’