I'd say if you want to spend all 3 hold at once to do 3-harm, you'd add bonuses once. So the implant syringe would make it 4-harm. And if you had Merciless (like my Brainer does), that would be 5-harm.
But if you wanted to spend each hold one at a time with mere seconds between them, add bonuses to each one. So, 2-harm for each hold with the implant syringe. And 3-harm for each hold if you also have Merciless.
And, you know, describe the difference in the fiction. Like: "I flood his system with everything, all at once." Or: "I hit him in the brain with my brain. Then I do it again. He dead? No? Fuck it, again in the brain. Howz 'e doin' now?"