[Like DvB said] Max Heart Rate is activity specific due to the muscles involved with the activity and is affected by the position of your body while performing the activity. For multisport, it's preferable to call the highest heart rate for each sport Peak Heart Rate.
Triathletes typically see a different 'peak' heart rate depending on whether they are running, cycling, or swimming. Running usually yields a higher peak heart rate than cycling by about 7 to 10 beats per minute. Cycling usually yields a higher peak heart rate than swimming by about 7 to 10 beats per minute.
To find your peak for each sport, whether self administered or supervised, some sort of maximal exercise test needs to be performed under circumstances that will yield an accurate snapshot of your current fitness level (i.e., fully rested, glycogen stores repleted, temperature/humidity/wind at optimal levels, etc.)
Once one knows their peak for each sport, correct training ranges can be calculated allowing one to focus on training the targeted energy system.