Much like a VNA, your rtl-sdr is down-converting from (say) 1.2Mhz to somewhere around DC. This is limited not by your sample rate but by the oscillator your rtl-sdr is mixing into your received signal and the general frequency profile of all the analogue bits in between.
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Your rtl-sdr is _sampling_ at a mere ~3Mhz.
Much like a VNA, your rtl-sdr is down-converting from (say) 1.2Mhz to somewhere around DC. This is limited not by your sample rate but by the oscillator your rtl-sdr is mixing into your received signal and the general frequency profile of all the analogue bits in between.