A bedikah performed after sunset is too late and does not count as a bedikah for that day of your shivah neki'im, so if you missed sunset there is no need to perform a bedikah even if it not yet tzeit hakochavim (nightfall).

Despite missing a bedikah on day 3, your clean days are still valid and you may continue your original count. While one should perform bedikot every day throughout the seven clean days, as long as a bedikah was performed on day one and day seven, the clean days are valid and you may immerse as planned.