I think making bass sounding fuller in a 3 piece band has less to do with gear and more to do with what & how you play, and what everyone else is playing. (i know. Commenting on TB and NOT recommending throwing money at the problem by buying more gear? What's wrong with me?)
Part of this is gonna be on the guitarist, which can be tricky getting another musician to change. Guitarists who are used to playing lead or soloing with rhythm guitar support might not realize they have to lead/solo differently when in a 3 piece. They have to rework their solos and lead lines to also provide harmonic support for themselves like a rhythm guitarist would while they solo. It's why a lot of times 3 piece bands lose all their energy right at the solo when everything should be exciting. They think the bass & drums aren't big enough, but what they really need is to change how they play, or hire a second guitarist.
There are also two different contrasting techniques you can use: Fill in more space by playing more, or be bigger by playing less.
Playing more bridges the rolls of the bass & the rhythm guitar. Not necessarily really busy bass-lines, but maybe more pedaling and driving when you can, and a few extra notes that help spell out the chords where appropriate. I'm thinking Rage Against the Machine or Cream.
Playing less and being bigger: Think of how huge simple bass-lines can be in hip hop. Just a few notes starting and stopping at exactly the right time can be WAY BIGGER than a lot of notes or chords. Think of "the drop" in electronic dance music. After the buildup and the break, one of the BIGGEST things i do in my productions are come back with just the Kick & Synth Bass (and maybe snare.) Everything else waits a measure. If there is too much it would sound smaller and less impactful. Less is bigger.
Then tonally, i would suggest a relatively even tone across the spectrum, tapering off up high. Aim for a fair amount of Lows & Low-mids. A healthy amount of High-mids but still favoring the two lower bands. And then highs equal too or less than the high-mids. Just fill up that sonic space, and then adjust to fit with the guitarist's tone and the rooms. I can't give specific EQ settings because different basses' and amps' natural frequency response curve will require different amounts of EQ to achieve that spread.
And then, maybe just an ever so slight touch of distortion. Either just hitting the sweet-spot right at gain-breakup on your amp, or some saturation, or an overdrive that's just barely breaking up but not full-on distortion or fuzz or anything. No distortion techniques that reduce the low-end. Just something that makes you go "whoa that's a fat bass!"
And then maybe, some slight compression as a thickening agent.
And then maybe, maybe turn the amp up a little louder. Louder is exciting to the ears (until it's too loud.) Louder is bigger, but you run the danger of getting into a volume war with the guitarist. Or the mix engineer will just turn down the bass if it's too loud. And also all the tone thickening from before will give the appearance of louder.