bmap <- leaflet() %>%
  addProviderTiles(providers$Stamen.TonerLines) %>%
  addProviderTiles(providers$Stamen.TonerBackground) %>%
  addPolylines(buffer_1, opacity = 1, col = "red") %>%
  addPolylines(buffer_2, opacity = 1, col = "blue") %>%
  addPolylines(buffer_3, opacity = 1, col = "green")

(AddTiles %>% does not seem to make a difference so I have left that out)

bmap <- leaflet(buffer_1) %>%
  addProviderTiles(providers$Stamen.TonerLines) %>%
  addProviderTiles(providers$Stamen.TonerBackground) %>%
  addPolylines(opacity = 1, col = "red")