Grant's contract at Adams Park expired at the end of last season and the speedy midfielder becomes Town's first signing ahead of their maiden Championship campaign.
He began his career at Watford, awarded a first professional contract in 2006, before joining Aldershot Town where he helped them secure promotion to the Football League.
The former Jamaica U20 international moved on to Crewe for a fee of £130,000 in 2008 and then joined Wycombe in 2011, scoring 14 goals in 68 appearances.
Defender Jones, released by Leicester, has agreed a deal to keep him at Huish Park until Christmas.
Jones started out at Arsenal before joining Leicester in 2010 and he has been a regular for Northern Ireland at youth levels.
And to complete the triumvirate, striker Hoskins makes the move to Somerset from Southampton.
Hoskins has been a regular in the Saints' U21 side and won goal of the season for his effort against Newcastle last season - he has also spent time on loan with Preston, Rotherham and Stevenage.