Using @Pavel's answer as guide, here's a helper method if you don't have the points but have start x,y and height and width. Also can draw inverted/upside down - which is useful for me as it was used as end of vertical barchart.
private void drawTriangle(int x, int y, int width, int height, boolean inverted, Paint paint, Canvas canvas){
Point p1 = new Point(x,y);
int pointX = x + width/2;
int pointY = inverted? y + height : y - height;
Point p2 = new Point(pointX,pointY);
Point p3 = new Point(x+width,y);
Path path = new Path();
path.setFillType(Path.FillType.EVEN_ODD);
path.moveTo(p1.x,p1.y);
path.lineTo(p2.x,p2.y);
path.lineTo(p3.x,p3.y);
path.close();
canvas.drawPath(path, paint);
}