Any special reason why you need an aggregate query? One way to do it would be to use relationships, like that:
SELECT Id, Name, (SELECT Id FROM CampaignMembers)
FROM Campaign
WHERE Id IN ('701U0000000MVoQ', '701U0000000MLFR', '701U0000000MVoL')
Later in code it can be accessed like that
List<Campaign> campaigns = [Id, Name, (SELECT Id FROM CampaignMembers)
FROM Campaign
WHERE Id IN ('701U0000000MVoQ', '701U0000000MLFR', '701U0000000MVoL')];
for(Campaign c : campaigns){
System.debug(c.Name + ': ' + c.CampaignMembers.size());
}