ACR MLG UNAUTH RWY ENTRY TAXI CAUSES UNK ACFT ON FINAL TO GO AROUND.

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THE PROB AROSE DURING OUR LNDG ROLLOUT ON RWY 3 AT CHARLESTON AFB; SC. DURING OUR ROLLOUT INSTRUCTIONS WERE ISSUED BY A TRNEE TWR CTLR AS TO WHERE WE WERE TO EXIT OUR RWY. DUE TO REQUIRED CALLOUTS IN THE COCKPIT BY THE PNF--SPOILERS EXTENDED ENGS IN REVERSE PLUS THE ASSOCIATED HIGH NOISE LEVEL WITH REVERSING ENGS THESE INSTRUCTIONS WERE ALL BUT INAUDIBLE. WITH THE ACFT SLOWED TO A SAFE TAXI SPD WE EXITED RWY 3 AT THE INTXN OF RWY 3 AND RWY 33-15 WHICH WAS WHAT WE INTERPRETED THE CTLR INSTRUCTIONS TO BE. AFTER OUR TURNOFF WAS COMPLETED WE OBSERVED AN ACFT ON FINAL FOR 33 APPROX 5 MI OUT. AS WE WERE UNABLE TO CLR 33 IN TIME; THE ACFT ON FINAL WAS INSTRUCTED TO GO AROUND WHICH WAS DONE. SHORTLY THEREAFTER; WE CLRED RWY 33 AND PROCEEDED TO THE PAX TERMINAL AT NO TIME ON FINAL FOR RWY 3 WERE WE TOLD OF ANOTHER ACFT LNDG ON RWY 33 OR TOLD TO HOLD SHORT OF OR NOT TURN OFF AT THE RWY 33 INTXN. ALSO CONTRIBUTING TO THIS PROB WERE THE SIMILARITY OF RWY #'S 3 AND 33; AND THE LOUNGE AMOUNT OF CONSTRUCTION TAKING PLACE ON THE ARPT AND TXWYS LEADING TO THE PAX TERMINAL. I FIRMLY BELIEVE INSTRUCTIONS SUCH AS THOSE WE WERE GIVEN BY CTLING AGENCIES SHOULD BE ISSUED AND CONFIRMED PRIOR TO LNDG ROLLOUTS SINCE ROLLOUTS ARE A VERY STRESSFUL; BUSY AND NOISY TIME INSIDE COCKPITS.