My Baby Jogger Summit XC is FANTASTIC (pushed forwards too!) on Gravel, Pebble Beach, Snow and Sand Dunes. I think it's the extra large rear wheels and all wheel suspension - along with very stable wide base and front wheel that locks forwards (if necessary) to extend the length of the wheel base that helps.
Be warned though, the Baby Jogger Summit XC is HUGE compared to most 3 wheelers - although it isn't too heavy at only 10.9kg - it's sturdy and packs down really small (with that magic pull handle fold you find on baby joggers for swift folding), and the wheels all pop off really easily in seconds so it takes up very little boot space in the car.
Aside from this buggy - the only other one I've tried that was truly brilliant on the beach was the Micralite Fast Fold (similar to the superlite model) with the addition of the "ATK" (all terrain kit wheels - now sadly discontinued and obselete and hard to find even on eBay) and handlebar extenders. However, you did miss out on the suspension and stability offered by the Baby Jogger Summit XC.
I've looked really hard at LOTS of 3 wheelers for use on Gravel / Sand/ Pebbly Beaches / In the Snow - including fairly deep snow - not just a bit on the pavement / In the Forest (very uneven, boggy, tree roots, and steep or hilly in places/ Moorland (including through heather and over rocks) / Mountains and Streams etc.... (because I really LOVE to do what I want to do in the countryside and not be hampered by lack of the right equipment to do it, and because I we as a family like to do whole days outings - so our youngest does get a bit worn out with all the fresh air and running around - and also the buggy is helpful to carry our supplies too...lots of drinks and food plus waterproofs if necessary). I also use this buggy for the school run - and for shopping daily (I don't drive just yet - although doing my test soon! but even when test is passed fuel is sooo expensive so I'll still have to walk mostly). This was the best to suit my needs and I'd recommend it to anyone else looking for a serious off-road buggy because it's just such brilliant value for money. Really durable and has a lot of great features for what you pay (typically it sells around £300 new), the bigger wheels - and better all wheel suspension than you get on your average off road buggy (the rear wheel suspension works both up and down and front to back - but NOT side to side - so no lateral wobble) and the front wheel also has fab and effective suspension - which is handy for kerbs in urban situations too. The fabrics are all great quality - and it looks really modern and sporty (I chose the stone/black version to be subtle in the countryside!), and the large canopy really keeps off rain and sun alike - I rarely need the plastic raincover - never bother taking it with me!
Sorry to go on and on - I really don't work for Baby Jogger - but I do just LOVE the amount of value you get in their products. Fabrics are guaranteed for 1y and the frame gets a life time guarantee (that's how confident they are that you'll have no problems!). 