Here are (I think) the replays shown in the last two seasons.
2014-15
R3 BT - (West Ham v Everton (BT), Ipswich v Southampton (BBC))
R4 BBC - (Bolton v Liverpool (BBC), Man Utd v Cambridge (BT))
R5 No Replay
R6 BT - (Blackburn v Liverpool (BT), Reading v Bradford (BBC))
2015-16
R3 BBC - (Leicester v Spurs (BBC), Bristol City v West Brom (BT), Liverpool v Exeter (BT))
R4 BT - (West Ham v Liverpool (BT), Peterborough v West Brom (BBC))
R5 BT - (Hull v Arsenal (BT))
R6 BBC - (West Ham v Man Utd (BBC))
It looks like one broadcaster gets R3/R6 and the other gets R4/R5 which makes sense given the probability of replays declines as the competition goes on. So it looks like it's BT that's lost out most on the scrapping of R6 replays (as I think they were due to get first pick of R3/R6 replays this season), so that might explain why they got an extra third round tie. It will be interesting to see if the BBC get a third game in next season's third round.