Reduce the number of monsters after assessing how optimised the party is. The minimum and maximum levels of optimisation and (especially) designed cooperation matter more than the number of players.
Adventures, especially early adventures are very particularly staffed with monsters. Some encounters are silly, others are very very hard. For a smaller group playing through an established adventure, encourage them to optimise their group (link1), (link2). You'll likely want to re-staff the monsters anyways from later sources, re-skinning as necessary to achieve awesome combats. Just include one or two fewer of these newly-upgraded monsters in the first tests.
A group which is designed to work together should require minimal tweaking from the adventures, save for the normal updating to the latest versions of monsters and other sanity checks. If I was running this for the group, I'd allow unlimited rebuilding of characters on long rests, with equipment by the rules (level-1, equal level, and level+1 magic items, with gold and common items totalling level-1 worth), and only worry about dropping plot-significant or narratively interesting items. That way, the group can adapt to challenges and, in a sense, auto-balance without needing excessive in-world fictive justification for those changes.
If it was my group, I'd probably recommend something like: (Ardent or Shaman), Warlord, Thief. By carefully ensuring that the Ardent and Warlord know how to provide the best setups to the thief, all three of them can do "better-than-thief" levels of damage with the MBA granting capabilities of the two leaders. In this instance, I would absolutely suggest that on a granted attack, if the granting player isn't doing anything, to allow the granting player to roll the dice, but that should be seasoned to taste. While more interesting combinations are possible here, this is a very simple setup that should make short work of the "default" difficulty, while providing a fascinating tactical and technical challenge to your players. (Working out the narrative of the group would also be fascinating and could be very very neat, but that's an answer to another question.) This level of optimisation is likely overkill, and so a fair chunk of "dialing in" will be necessary to figure out the exact comfort level of the group. The offloading of items onto the players is mainly a way of making it so you don't need to worry about boring build-specific items, and so you can focus on wonky/interesting narrative items which reflect the impact of the character's actions on the world, rather than obligatory items to keep their math correct.