There is no dodge test for Grenades. It's a straight up single threshold test. WHether you're throwing at ground or at a runner, the test is the same. It's a THrowing + Agility [Physical] (3) test.
There are no rules for catching or throwing a Grenade back yet. Those are traditionally in the first Combat expansion, and I have no reason to think they won;t be this time around.
Grenades only stack a little. You get +1/2 DV for the second grenade only. You get no extra DV for grenades past the second. You get -1 AP per grenade past the first.
So three HE Grenades hit the same location and go off on the same action (16P, -2 AP base damage). They will do a combined damage of 24P with a -4 AP.
Grenades are dangerous. Also remember that they can be set off by fun things like fireballs.
Bull
Then what does "This method uses the standard Ranged Attack rules but adds an extra step if it misses the target (no net hits on attack roll)" (SR5O 181) mean under the entry for Motion Sensor? Especially when special grenades rules are listed before in the same section? If you never target a person with a grenade, then how does one get the Targeted by an Area-Affect Attack defense modifier which stats "Apply a -2 modifier when trying to defend against weapons like spells, grenades, rockets, or missiles with a blast or area effect (SR5O 190)" for things other than spells? Rockets and missiles fall under the same heading as grenade launchers so they full under the same rules.
I will admit that the grenade stacking only applies to two grenades because it explicitly says "when two explosions occur on the same Combat Initiative Score and both blast effect the same character add half the value of the lower DVs to the highest DV and apply it all as a single modified Damage Value for the purposes of Damage Resistance tests. For AP calculations, use the best AP and improve it by 1 for every additional explosion (SR5O 183)." That means technically a lot of stuff could happen when three or more grenades blow. They could merge together like you say, you could start merging them so the first grenade adds to the second which then adds to the third, or they could pair off (so it's one 24P blast and a second 16P blast).
Honestly, grenades and other explosives have enough drawbacks that they aren't the go-to weapon for all situations. It's just that two grenades are a huge deal and need to be factored into all combats particularly why they aren't grenades used by either side this fight. There's also Flash-Bang which do the still respectable 10S -4 AP and have far fewer drawbacks and are restricted legality. They don't even have drop off due to range so you could multiple attack with them and a bit of missing won't matter.
I do think there ought to be a "dive for cover" interrupt action that gives you some movement "early" for this and other AE attacks. My off the cuff thought is -10 Initiative Score, allows you to move your agility in meters (or just 1-2 meters) and makes you prone. Hit the Dirt is close, but just leaves you in place.
