this is my opinion...
its better you extend early next season...you still have 7 day to do that..no meaning your sign all of them until 3 years...because next 3 season their rating was keep down. maybe this season 4 star but next season will became 3 star.. Yes you can train your player but its still not enough to carry until the next 3 season..if you keep training its still impossibble to achieve 5 star player for next 3 year.. you still need to buy another player..if you sort of player just sign all of them...but sell or sack player more then 32 years old..normally they will retired at age 33 or 34..(some reserve player retire at 32yrs...maybe sad can't play more matc d)
example: currently you at season 3....then you extend your 5 star player..then at season 4 their rating was down to 4 star..you can train to increase their rating but still not enouh to keep this player until season 5...not all day you can train them especially if they injured..normally you can carry forward for next 1/2 season...then sell and get buy new 1..its more quit to stronger your team compare to train them..
good luck...