Skytrain from Jewel to SQ First Class Reception at Changi T3?
 

Is there a landside Skytrain that goes from the Jewel to near Singapore Airlines First Class Reception at Terminal 3 of Changi Airport?

No. It is just a 5-10 mins walk. N I think there are travelator as well.

as sbs2716g mentioned, there are travelators from jewel to T3. there are skytrains from T1/Jewel to T3. both are covered indoors

both travelators and skytrain gets to the rightside of terminal 3 (as you can see from this map), and you have to manually walk to the left side of T3

https://www.changiairport.com/en/map.../103.98803/-67

that landside&airside T3 skytrain picks up from the northside of T1 (which is connected to jewel), but its still a decent walk from jewel
https://www.changiairport.com/en/map...103.989389/-67

that landside&airside T3 skytrain also goes through (but does not stop at )Jewel, then picks up at T2. the view is pretty good
https://www.changiairport.com/en/map...103.990038/-67

The other way is to take the Skytrain from T1/Jewel to T2 (which is the closest skytrain to Jewel), then pass through immigration in T2 and then take Skytrain from F end of T2 to A end of T3, which is quite close to the SKL/F lounge entrance.

But if you must, you could always download the Grab app and pick up a Grab in the basement of Jewel/T1 to take you to the First Class Reception in T3 :D At the moment quoting $6 for that ride, but could be higher during peak hours.