import pygame
from pygame.locals import *
from sys import exit
pygame.init()
screen = pygame.display.set_mode((1200, 800), 0, 32)
points = []
while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            exit()
        if event.type == MOUSEBUTTONDOWN:
            points.append(event.pos)
    screen.fill((0,0,0))
    if len(points) >= 3:
        pygame.draw.polygon(screen, (255,0,0), points)
    for point in points:
        pygame.draw.circle(screen, (0,0,255), point, 5)
    pygame.display.update()