Data not getting inserted into Timestream table and how to know the dimensions and value

  MQTT_MAX_PACKET_SIZE in PubSubClient.h
  char fakeData[128];

     float var1 =  random(55,77); 
     float var2 =  random(77,99);
     float var3 =  random(99.55);
     sprintf(fakeData " 
    {\"uptime\":%lu,\"LDR\":%f,\"GPS\":%f:\"USS\":%f}", millis() / 1000, var1, var2,var3);

i want to send 3 data to aws cloud console through mqtt so what dimension and value to give