They have been wonderful notes, but not a song. Beautiful tiles, but no mosaic. Terrific scenes, but no movie.

The success of Greg Bird in 2015, Gary Sanchez in 2016 and, now, Aaron Judge in 2017 created buzz and elevated optimism. What would it look like if it all came together at one time?

After all, one key component of what made the Cubs — the Yankees’ opponent this weekend at Wrigley Field — the champion Cubs was that their core of young position players emerged rather simultaneously to form a symphony of precocious offense. Even now eight of the nine Cubs with their most plate appearances are 27 or younger.

In spring training 2016, standing in a press box watching a game together, I asked Cubs president of baseball operations Theo Epstein to tell me something interesting. Without hesitating, he said, “Young good, old bad.” Simple. His team then went on to win its first title since 1908 and did it with eight players 26 or younger accumulating at least 200 plate appearances, tied with the rebuilding Phillies for the most in the majors.

The Yankees literally reached into the Cubs’ overflow basket of young hitters to address this issue, acquiring Starlin Castro (who only turned 27 in March) in the 2015-16 offseason and their now-top prospect, Gleyber Torres, at last year’s trade deadline.

The two pitchers traded for those players, Adam Warren and Aroldis Chapman, are Yankees again and have arguably been their two most effective relievers in an AL East-leading 17-9 start. The only times that Bird, Sanchez and Judge have played together were the first four games this season and the Yanks went 1-3 as Bird and Sanchez struggled, and Judge was not yet demonstrating that he was going to be the biggest — in every way — story in the game.

Sanchez is due back this weekend against the Cubs, but Bird is on the DL again.

Getting a group of young stars healthy and thriving concurrently is difficult. The Mets, for example, have never had Jacob deGrom, Matt Harvey, Steven Matz, Noah Syndergaard and Zack Wheeler take even one turn together, and with Matz and Syndergaard on the DL, the possibility grows that it will never happen.

The fragility of pitching makes it more difficult than with position players, but Bird has, to date, shown himself to be brittle as well. He missed all of last season following shoulder surgery. But he temporarily allayed concerns by doing in spring what Judge has done in the regular season — hitting .451 with eight homers. But he fouled a ball off his right ankle late in spring training and the injury has just not healed, leading to a 6-for-60 (.100) showing, being sidelined for five days earlier this year and now a DL stint.

The Yanks hope Bird will return in two weeks, but they are not sure how he will heal from the bone bruise. This is somewhat reminiscent of their life with Nick Johnson, who like Bird was a lefty-hitting first baseman with on-base skills and power. But Johnson just kept breaking down, healed slowly and never fully expressed his skills for elongated periods.

Bird is the Yankees’ best first base prospect since Johnson, and his 11 homers in 46 games helped the Yankees earn a 2015 wild-card spot. He was not around last season when Sanchez scintillated with 20 homers in 53 games. And both have been mostly bystanders as Judge has climbed to the major league lead in homers with 13.

Brian Cashman had said he thought the Yankee offense would be better this year than last by simply replacing the faded Alex Rodriguez and Mark Teixeira with the younger players. In the case of Judge, that is true, as he has been akin to a one-man wrecking crew. But it should be noted that Castro, Didi Gregorius and Aaron Hicks are also in just their age-27 seasons, and hinting this might be their best campaigns yet in helping the Yanks to the AL lead in runs.

For now and the near future, though, the Yanks were banking on Bird, Sanchez and Judge forming their middle-of-the-order might, with hopes of Torres and Clint Frazier – both of whom have heated up in the minors – soon joining the band. After all, if you combined the ’15 of Bird, the ’16 of Sanchez and the ’17 of Judge, then in 124 games and 446 at-bats there would be a slash line of .291/.376/.641 with 44 homers and 115 RBIs.

But we haven’t seen them do this as a group.

Timing is everything – also difficult.