No. Austin also has a $5M roster bonus in 2018. And when is his 2017 roster bonus due? Even if you wait until the Rams pay it you're looking at $9M a year (after this year) with a lot guaranteed. You can't cut him after this year due to all the guaranteed upcoming salary. This is a non starter.
There seem to be some discrepancies between how Overthecap and Spotrac present Austin's contract details.
According to Spotrac, Austin's 2018 roster bonus ($5m) is due 3/11/18 (OTC doesn't have those details), so, the Pats would be on the hook for that + Austin's $3m salary in 2018, if a roster bonus transfers with a contract.
Also according to Spotrac, with a post-June 1 CUT or TRADE of Austin, the Rams incur a 2017 dead money cap charge equal to the 2017 roster bonus, which leads me to believe that the $11.47m roster bonus for this season was due/"paid" in early March of this year. However, Spotrac also shows the Rams incurring a 2018 dead money cap charge equal to the 2018 roster bonus, if they trade Austin in 2017 (after June 1st). Now that I think about it, the Rams eating the 2018 roster bonus doesn't make any sense.
OTC shows the same result for a post-June 1 CUT (the Rams eat Austin's 2017 AND 2018 roster bonuses). However, under TRADE, it shows the Rams having ZERO dead money for either year, and it doesn't distinguish between pre-June 1 and post June-1. So that's the discrepancy.
In any case, both sites agree that Austin' 2019-2021 salaries average $9.4m, but also agree that nothing is guaranteed after 2018.