SapphireMemories
Michief Maker.
Based on my Pokémon's stats in the demo the Mega Evolutions have roughly these base stats;
Mega Sceptile
HP: 70 --> 70 (+0)
ATK: 85 --> 105 (+20)
DEF: 65 --> 75 (+10)
SPA: 105 --> 140 (+35)
SPD: 85 --> 85 (+0)
SPE: 120 --> 145 (+25)
Mega Swampert
HP: 100 --> 100 (+0)
ATK: 110 --> 150 (+35)
DEF: 90 --> 105 (+15)
SPA: 85 --> 95 (+10)
SPD: 90 --> 105 (+15)
SPE: 60 --> 70 (+10)
Mega Glalie
HP: 80 --> 80 (+0)
ATK: 80 --> 115 (+35)
DEF: 80 --> 80 (+0)
SPA: 80 --> 115 (+35)
SPD: 80 --> 80 (+0)
SPE: 80 --> 100 (+20)
As I don't know my Pokémon's IVs I can't be entirely sure and am missing 10 points from each but its a good rough answer.
Mega Sceptile
HP: 70 --> 70 (+0)
ATK: 85 --> 105 (+20)
DEF: 65 --> 75 (+10)
SPA: 105 --> 140 (+35)
SPD: 85 --> 85 (+0)
SPE: 120 --> 145 (+25)
Mega Swampert
HP: 100 --> 100 (+0)
ATK: 110 --> 150 (+35)
DEF: 90 --> 105 (+15)
SPA: 85 --> 95 (+10)
SPD: 90 --> 105 (+15)
SPE: 60 --> 70 (+10)
Mega Glalie
HP: 80 --> 80 (+0)
ATK: 80 --> 115 (+35)
DEF: 80 --> 80 (+0)
SPA: 80 --> 115 (+35)
SPD: 80 --> 80 (+0)
SPE: 80 --> 100 (+20)
As I don't know my Pokémon's IVs I can't be entirely sure and am missing 10 points from each but its a good rough answer.