Is it true that whoever misses more than three Friday prayers will have his heart sealed, and is the Dhuhr prayer sufficient for him, taking into account that he is a teenager and does not have transportation to the Friday mosque?

Using the phrase "تحياتي" (my greetings) to mean "سلامي" (my salutations) is permissible, but the Sunnah (definition) is to say "السلام عليكم" (peace be upon you). As for missing Jumu'ah prayer three times or more without a valid excuse, it is considered one of the major sins and leads to Allah sealing the heart of the one who misses it and making him among the heedless. This warning applies to the adult, capable, resident within the country, or one who hears the call. However, the adolescent who has not reached puberty is not obligated to attend, and the warning does not apply to him. And whoever is not required to attend Jumu'ah prayer should pray Dhuhr as four rak'ahs.