This weekend marked the close of Boston Ballet’s third season under artistic director Mikko Nissinen, and the change in the quality of the company’s performance over that time has been striking. “Without the kind of wholesale purge that could have left the Ballet reeling, [Nissinen] has slowly changed the company’s personnel both behind the scenes and onstage. The coaching staff he’s brought in — especially Spanish-born Trinidad Vives — is first-rate. Those additions have largely replaced the Russian teachers and coaches whose attitude was that their dance heritage alone made them superior.”