Hun Sen said Article 78 of the constitution does not allow the prime minister and the king to dissolve the parliament, so he can't dissolve parliament and he cannot step down:
1. The constitution never allow co-prime ministers and co-ministers in 1993, but after losing the election, Hun Sen refused to step down and forced the constituent assembly to write a constitution stipulating about the co-prime ministers.
2. The constitution never allows a coup d'etat but in 1997 Hun Sen mounted a coup d'eta against the election winner to become the sole prime minister, with a puppet prime minister chosen by him. After the 1998 election, which he cheated to win, he changed the constitution abolishing the co-ministers and "co-prime ministers" to make himself the sole prime minister.
3. The constitution stipulates that Cambodia is a democracy with a pluralistic/multi-party system, but Hun Sen forced the king to inaugurate a single-party parliament after the July 28, 2013 election.
Hun Sen's actions above were/are illegal and unconstitutional, so, this time, dissolving the parliament or stepping down from power won't be illegal or unconstitutional because Hun Sen lost the election and came to power illegally and also the people/voters demanded that he step down.
Hun Sen's last sentence of "beware of a third hand" is a threat that he could hire thugs or organise his hit squads to cause trouble at the protests or assassinate protesters and CNRP leaders, like Sam Rainsy and Kem Sokha, and blame it on a "third hand", a third party, but was organised and hired by him or his men.