tests/fixture/txsources: add initial balance for validator-3

[ptrus]

Forgot adding it in: #2815

[codecov]

Codecov Report

Merging #2830 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2830      +/-   ##
==========================================
- Coverage   67.23%   67.07%   -0.16%     
==========================================
  Files         342      342              
  Lines       32968    32968              
==========================================
- Hits        22165    22113      -52     
- Misses       8121     8161      +40     
- Partials     2682     2694      +12     

Impacted Files	Coverage Δ
go/consensus/tendermint/api/api.go	73.58% <0.00%> (-15.10%)	⬇️
...ompute/txnscheduler/algorithm/batching/batching.go	78.66% <0.00%> (-6.67%)	⬇️
go/common/cbor/codec.go	64.70% <0.00%> (-5.89%)	⬇️
go/runtime/tagindexer/tagindexer.go	68.47% <0.00%> (-4.35%)	⬇️
go/consensus/tendermint/roothash/roothash.go	65.54% <0.00%> (-3.38%)	⬇️
go/storage/metrics.go	88.04% <0.00%> (-3.27%)	⬇️
go/runtime/transaction/transaction.go	75.90% <0.00%> (-2.41%)	⬇️
go/worker/compute/txnscheduler/committee/node.go	64.50% <0.00%> (-1.86%)	⬇️
go/storage/database/database.go	79.09% <0.00%> (-1.82%)	⬇️
go/worker/common/p2p/p2p.go	64.41% <0.00%> (-1.81%)	⬇️
... and 10 more

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