Without reducing the number of enemies, you can make the fight easier by manipulating the following factors:
Surprise
With 4 Goblins and only 3 characters, getting a surprise round will swing the direction of the encounter wildly, depending on who's surprised whom. As the goblins are waiting in ambush, it is highly unlikely that the PCs can surprise them so the best you can do is make sure you communicate the dangers well. Ask for Perception checks instead of using Passive Perception, if a PC's passive perception is really bad (less than the Goblin's stealth roll). I know this isn't RAW but you should be lenient with such a small party.
This is important because a Goblin can shoot down most characters (especially a spellcaster) with a hit and a good damage roll (let's leave criticals out because reasons...) If the goblins gets an extra round to shoot their arrows, it's going to result in a bloodbath for the PCs.
Terrain
Another factor that the DMG doesn't take into account when calculating an encounter's difficulty is Terrain. Make the terrain in favor of the PCs and less favorable to the Goblins. Perhaps the dead horses can provide cover, perhaps the supply wagon are in between the party and the goblin archers negating the ranged threat while the PCs kill the goblins in melee. Though the Goblins can hide as a Bonus Action, make them stay within reach of the party, 30 feet from one member should be enough for that party member to move without having to use an Action to Dash (except if he/she has less than 30 feet of movement).
This is the part where I spout some unconventional wisdom. The above already covers some good points and you can use that to make the encounter bearable already.
Increase the PC's starting level
If you plan on going through the whole starter set with just the 3 players, consider increasing their starting level to 2. It doesn't do much for the game except make the first dungeon crawl and the first encounters a bit easier (but still pretty difficult, mind you). This way, you do not have to scale down each and every encounter in the Goblin Hideout to accommodate the reduced party size.
You may also want to check out my answer to this question. I also outlined some alternative ways to strengthen the PCs there, you can use all those except number 2 (well, that depends on you if you want to give them magic items at the start) in the starter set, too.
