[provigator]

окей,

function mask() {
	ctx.drawImage(pic2, 0, 0)
	ctx.globalCompositeOperation = 'destination-in'
	ctx.drawImage(pic3, 75, 0)
	ctx.globalCompositeOperation = 'destination-over'
	ctx.drawImage(pic1, 0, 0)
	ctx.globalCompositeOperation = 'source-over'
}

работает, буду разбираться...