Similar to winter, if ambient temperature gets to hot efficiency starts to decrease. The sweet spot for maximum range is between 21c & 22c, after 25-26c range starts to slide downwards sharply. However, at the sweet spot of21-22c, you often get upto 130% of range, so the drop in range at hotter temperature isn’t quite, as noticible as you see in winter. Other factors also come into play such as acceleration, speed, head winds etc.
At 3300 miles, it’s unlikely the battery has degraded to a noticeable level (stranger things have happened though) and think my first call would be resetting the range computers, as a try and see.
Also, as a ballpark firgure @3,5mkwh x 77kWh battery = 269.5 miles, so you are getting a bit more than 3.5mkWh