* 18.2 percent return from infrastructure (Adds comment on strategy, details on asset classes): TORONTO, April 3 (Reuters) - Ontario Teachers’ Pension Plan, Canada’s third-biggest public pension plan, more than doubled its 2017 net return from investments compared with the year before, boosted by strong returns in private equity and infrastructure. The fund, which administers pensions for 323,000 working and retired teachers in Canada’s most populous province, reported a net return from its investments of 9.7 percent in 2017, compared with 4.2 percent in 2016, when unfavorable currency movements had an impact.