Canon is now selling the easily-lost covers for the new multifunction shoe (initially, the cover for the R3 was available only as a replacement part through Canon Service).
For the R3: Canon ER-SC1 Shoe Cover B&H | Adorama | Canon USA
For the R7 / R10 / R5 C: Canon ER-SC2 Shoe Cover B&H | Adorama | Canon USA
The R3's version is slightly more expensive ($12 vs. $8), because it's a cover that is part of the water resistance, rather than more of an insert like the one for the R7/R10/R5 C.
Personally, I've already misplaced the cover for my R3 (admittedly, I haven't looked very hard for it yet). I'll order a couple of them to keep on hand.
For the R3: Canon ER-SC1 Shoe Cover B&H | Adorama | Canon USA
For the R7 / R10 / R5 C: Canon ER-SC2 Shoe Cover B&H | Adorama | Canon USA
The R3's version is slightly more expensive ($12 vs. $8), because it's a cover that is part of the water resistance, rather than more of an insert like the one for the R7/R10/R5 C.
Personally, I've already misplaced the cover for my R3 (admittedly, I haven't looked very hard for it yet). I'll order a couple of them to keep on hand.