On the right of the church there is the elevated bit you can climb on. There is also an easier way of doing three kills. Pick up the vial and get out while two guards have a chat. You can easily access it with Cooper by climing on a shed, jumping over to a carriage and then balcony.
Originally posted by zzZJimmyBoy:how did you get the poison? It's on a table outside Marge's room (where there are two guards).