First, don't replace anything yet. Throwing money and parts at a problem eventually may solve it but it ain't efficient.
Get a voltmeter and get us some voltages:
Battery + terminal: battery before cranking, battery while cranking, battery after cranking.
Starter solenoid terminal 30: before, while, after.
Start engine, run up to 3000 rpm, measure battery + voltage.
Repart back, and we'll be able to narrow the possibilities down considerable...