The code you pasted doesn't make sense. The following should be fine:
engine.pushContext();
engine.evaluate(script1);
engine.popContext();
engine.pushContext();
engine.evaluate(script2);
engine.popContext();
engine.pushContext();
engine.evaluate(script3);
engine.popContext();
engine.pushContext();
engine.evaluate(script1);
engine.popContext();
engine.pushContext();
engine.evaluate(script2);
engine.popContext();
engine.pushContext();
engine.evaluate(script3);
engine.popContext();
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You don't "unload" scripts. When a script has finished executing you don't have to do anything more with it.
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