IPhone : How to create a json array from these json objects in iOS

I have a dictionary that looks like this.

NSDictionary *dict = @{@"cpu" : self.proData ,
                           @"date" : timeMilliString,
                           @"memory" : self.memData,
                           @"battery" : self.batData,
                           @"deviceID" : @"1"
                          };

I'm creating json objects from it like this:

self.jsonObj1 = [NSJSONSerialization dataWithJSONObject:dict options:NSJSONWritingPrettyPrinted error:&error];
self.jsonObj2 = [NSJSONSerialization dataWithJSONObject:dict options:NSJSONWritingPrettyPrinted error:&error];
self.jsonObj3 = [NSJSONSerialization dataWithJSONObject:dict options:NSJSONWritingPrettyPrinted error:&error];

Afterwards I want to put the 3 json objects into a json array and then put that json array into another json object. So finally, it should look something like this:

{"mobileData":[
        {   "cpu":-991,
            "date":"142441374.5834",
            "memory":978,
            "battery":-96,
            "deviceID" : 2
        },
         {  "cpu":-51,
            "date":"142441374.5834",
            "memory":978,
            "battery":-96,
            "deviceID" : 2
        },
        {  "cpu":-51,
            "date":"142441374.5834",
            "memory":978,
            "battery":-96,
            "deviceID" : 2
        }

    ]
}

I have tried it as follows but it's not working.

NSArray *jsonArray = @[self.jsonObj1, self.jsonObj2, self.jsonObj3];
NSDictionary *dict = @{@"mobileData" : jsonArray};
if ([NSJSONSerialization isValidJSONObject:dict])
        {
            // Serialize the dictionary
            json = [NSJSONSerialization dataWithJSONObject:dict options:NSJSONWritingPrettyPrinted error:&error2];

            // If no errors, let's view the JSON
            if (json != nil && error2 == nil)
            {
                NSString *jsonString = [[NSString alloc] initWithData:json encoding:NSUTF8StringEncoding];


                NSLog(@"JSON: %@", jsonString);
                //[jsonString release];
            }
        }

I'm getting an exception.

Any ideas? Thanks.