Syndergaard is estimated to make $5.9M in 2019. In his "third year of arb" as you say — which is misleading because he's got another immensely valuable year of team control after that — he'd probably make $10-12M if he stays healthy. It also doesn't really matter, because he's one of the five best pitchers in baseball if he's healthy. He made $3M last season and was worth $30M in surplus value despite pitching 150 innings.
"Robbing Peter to pay Paul" is pithy cliche but it's inaccurate here. I like the idea of trading the cheap outfielder because although his production is valuable, that value is something we're more able to pay for while other teams can't, and more able to develop relative to our other needs. Syndergaard+Gonzalez or +Pollock or, hell, even +Markakis is more valuable than Benintendi+Porcello, and it'd be a smart move to have Syndergaard under contract through 2021 so we don't have to drop $150M on Sale/Cole/Bumgarner next offseason.