It is good you opened with a statement you are buying your first TV box.
That is because from the proceeding questions and your understanding of TV boxes - others will be very reluctant to offer assitance.
I also hesitated a bit because you don't seem to understand the two types of TV boxes, as I also read you are considering LongTV in another post.

I will try to explain as short and concise as possible.

Buying Tx3 or Tx6, you are merely buying the hardware. The software (app) required to watch the stuff you require is available freely.
TVB drama - No issue of buffering majorly, and lots of free apps offer them. 720p mostly, 1080p also available depending on app.
Malaysia Free to View Channels - also no issue, a few ways to get it. Buffering not much, but availability depends on other factors.Also can use Unifi Play TV to get this. No 1080p as far as I know for these sources.

Tx3 vs Tx6 difference
- different processor (Tx3 has better, stable processor and cooler box)
- Tx6 has wifi Antenna (good if you can't connect via LAN cable, or your surroundings has a lot of neighbour WIFI routers)

Both Tx3 or Tx6 are popular. Be aware, Tx6 doesn't have root switch if you want to use AstroGo.
My recommendation is Tx3 if can connect LAN cable, Tx6 if you have lots of wifi interference.
Get Tx3 if you promarily want AstroGo. Tx6 will need to unroot manually (see if seller can help)

as for LONGTV, this is a diffferent beast altogether. You cannot compare LongTV against Tx3 or Tx6.
With LongTV, you are not just buying the hardware - but also hidden in the sale is an understanding that you will also get to install special apps later that will allow you to watch copyright stuff. Normally no buffering issue with them unless it is EPL night.

If you wish to compare then a better way to compare is
Long TV versus Tx3 + Subscription to black market lifetime IPTV (which we shall not discuss)
another way to compare is LongTV versus other boxes that similar to it in terms of offerings.
EV***, UnB****, and a number of other boxes that just recently introduced into Malaysia market.

A long time ago, about 3-5 years, senior peers would not recommend these boxes. Our recommendation has always been  to avoid them. Just buy the hardware, and subscribe to short term packages. Nowadays they changed tune seeing that some of these boxes could last pretty long, their recommendation is consider those with at least 3 years history in the market already. Again, the risk is all yours.

Addendum :- Sorry, you are not considering LongTV but EVpad for RM349. They are same type and category of boxes. Difference between them you would have to ask others who has experience using them both. There are subtle differences to which only frequent users will notice.