CARACAS, Aug 30 (Reuters) - Venezuela"s principal opposition parties are expected this week to announce candidacies for the upcoming elections for governors and mayors, three sources with knowledge of the situation said, following three years of election boycotts. Adversaries of President Nicolas Maduro sat out the 2018 presidential vote and the 2020 legislative election on the grounds that the proceedings were rigged in favor of the ruling Socialist Party. Opposition leaders in Venezuela"s provinces have been pushing to abandon the abstentionist strategy and have spent weeks in talks to forge a unified platform against Maduro allies. An announcement about joining the Nov 21 election for control of 23 states and 335 municipalities is expected on Tuesday or Wednesday, according to one of the sources, who asked not to be identified. Political parties Popular Will, First Justice, Democratic Action and A New Time are expected to field candidates, the sources said. The National Elections Council on Saturday extended the deadline for registering candidates until Wednesday, giving the opposition several additional days to complete their internal negotiations. The opposition last participated in elections in a 2017 vote for governors. Maduro allies and opposition leaders on Friday will resume talks in Mexico City aimed at easing a longstanding political impasse.